- Email: [email protected]
CH4 mixture
Thin Solid Films 547 (2013) 146–150
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Inductively coupled plasma reactive ion etching of magnetic tunnel junction stacks using H2O/CH4 mixture Tea Young Lee, Il Hoon Lee, Chee Won Chung ⁎ Department of Chemical Engineering, Inha University, 253 Yonghyun-Dong, Nam-Ku, Incheon 402-751, Republic of Korea
a r t i c l e
i n f o
Available online 24 April 2013 Keywords: Magnetic random access memory Magnetic tunnel junction stack Inductively coupled plasma reactive ion etching H2O/CH4/Ar
a b s t r a c t Magnetic tunnel junction (MTJ) stacks patterned with hard masks of 90 × 90 nm2 were etched and the etch characteristics were investigated using inductively coupled plasma ion etching (ICPRIE) in a H2O-based gas mix. As the H2O concentration in H2O/Ar mixtures increased, the etch profile of MTJ stacks improved and the redeposition decreased. Field emission transmission electron microscopy revealed that etching of the MTJ stacks in H2O plasma was stopped on a MgO barrier layer on which heavy redeposition occurred; however, the addition of CH4 gas to H2O solved this issue. Specifically, as the CH4 concentration in the H2O/CH4 gas mixture increased, the etch profile became more vertical and the redeposition was reduced considerably. Overall, the etching of MTJ stacks with a high degree of anisotropy without any redeposition was accomplished using a H2O/CH4 gas mixture in an ICPRIE system. © 2013 Elsevier B.V. All rights reserved.
1. Introduction A variety of memory devices have recently been developed to meet the demands of current smart generation. With the growth of smart technologies, the demand for improved memory devices that are non-volatile and have high-density storage and high speed is increasing. To date, dynamic random access memory (DRAM) and a few flash memories have partially satisfied this demand. However, these have many limitations that prevent them from meeting current requests under smart generation. Therefore, many intensive studies have been conducted to develop alternative memory devices including magnetic random access memory (MRAM), resistive random access memory, and phase-change random access memory [1,2]. MRAM is highlighted as a universal device for the next generation of memory because of its high density, rapid access, low power consumption, and non-volatile properties. MRAM devices consist of a complementary metal-oxide semiconductor and magnetic tunneling junction (MTJ) stack that corresponds to a capacitor of DRAM [3,4]. The MTJ stack is a key portion of the MRAM device. MTJ stacks have various ferromagnetic layers that contain Ni, Fe and Co for the induction of large magnetoresistance. These are composed of two ferromagnetic electrodes that have an insulating barrier such as oxides between them. The flow of tunneling current through the electrodes determines the relative magnetization orientation. The change in resistance between the high resistance antiparallel and the low resistance parallel alignment is characterized as tunnel magneto-resistance, which is an essential operating principle in MRAM devices [4–6]. ⁎ Corresponding author. Tel.: +82 32 860 7473; fax: +82 32 872 0959. E-mail address: [email protected] (C.W. Chung). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.04.022
The etching of MTJ stacks is prerequisite for integration of high density MRAM devices. The etching of MTJ stacks with nanometer patterns is known to be tremendously difficult because the metal films in MTJ stacks rarely react with chemically active species in plasma. Conventional etching methods such as ion milling and reactive ion etching have shown some problems including slow etch rate, redeposition on the sidewalls of patterns, poor etch profile and damage during the etching of magnetic materials and MTJ stacks. Inductively coupled plasma reactive ion etching (ICPRIE) has been applied to etch MTJ stacks and magnetic layers using Cl2/Ar, BCl3/Ar, and HBr/Ar gases [7–14]. However, the etch results were not satisfactory and these etching gases were very toxic and corrosive. A few investigations of the etching of MTJ stacks using CO/NH3, CH3OH and CH4/O2/Ar gases have been reported [15–18], and these gases were shown to be non-toxic and to pose no hazards to humans or the environment. Many studies to solve these etching issues have been conducted in attempts to develop etching gases and etch processes. In this study, H2O was used as an etching gas to investigate the etch characteristics of MTJ stacks patterned on a nanometer scale using an ICPRIE system. The etching of MTJ stacks was investigated by varying the H2O concentration in H2O/Ar gas. In addition, the addition of CH4 gas to H2O was proposed to improve the etch results obtained using only H2O/Ar gas. This H2O/CH4 gas mix had positive effects on MTJ etching when compared to the etch results obtained when only H2O/Ar gas was used.
2. Experimental details The MTJ stacks used in this study consisted of various magnetic and metal thin films and tunneling barrier layers. The MTJ structure
T.Y. Lee et al. / Thin Solid Films 547 (2013) 146–150
147
(b)
(a)
W/TiN W/TiN
MTJ
MTJ TiN TiN 100nm
(d)
(c)
W/TiN W/TiN
MTJ
MTJ
TiN
TiN
(e)
(f)
W/TiN MTJ TiN
W/TiN MTJ TiN
Fig. 1. FESEM micrographs of MTJ stacks etched at different H2O concentrations in H2O/Ar gas: (a) before etching, (b) pure Ar, (c) 10% H2O/Ar, and (d) 30% H2O/Ar, (e) 70% H2O/Ar, and (f) 100% H2O.
was W (70)/TiN (100)/Ru (5)/CoFeB (2)/MgO (0.8)/CoFeB (1.5)/Ru (0.8)/CoFe (1.5)/PtMn (15)/TiN (45)/oxide (nm). W and TiN layers were deposited on the tops of MTJ stacks as a hardmask and MgO thin film was employed as a tunneling barrier layer. All films were deposited on silicon-oxide coated Si substrate using a dc-magnetron sputtering method. The hardmask layers, which were composed of W of 70 nm and TiN of 100 nm, were patterned by e-beam lithography using a negative electron (e)-beam resist and then etched by ICPRIE in a Cl2/C2F6/Ar gas mix. After etching of the hardmask, the e-beam resists were stripped off using a stripping solution. Eventually, patterns of a 90 × 90 nm 2 square array that had spaces of 90 nm between each square were prepared on the MTJ stacks. An ICPRIE system (A-Tech System, Korea) was used to etch the MTJ stacks using H2O/Ar gas. The system was composed of a main chamber and a load lock chamber. The samples of MTJ stacks were loaded into the load lock chamber and then inserted into the main chamber. In the main chamber, a susceptor was maintained at the appropriate temperature of 12–15 °C using a cool fluid circulation system and the substrate was then cooled by filling the space between the substrate and susceptor with He gas. The main rf coil, which could supply rf
power at 13.56 MHz, was located at the top of the main chamber to generate a highly dense plasma. The other rf power at 13.56 MHz induced dc–bias voltage to the substrate to adjust the energy of ions in the plasma. The liquid H2O was evaporated in an evaporator at over 100 °C, which was sufficient to produce a flow of H2O vapor. The flow line was heated to prevent the evaporated H2O vapor from condensing in the line. The flow rates of H2O were controlled by an exclusive mass flow controller for H2O. H2O/Ar and H2O/CH4 gas mixtures were employed as the etch gases. The etch characteristics of the MTJ stacks were investigated by varying the H2O concentration in H2O/Ar gas and the CH4 concentration in H2O/CH4 gas. The etch profiles of the MTJ stacks were observed by field emission scanning electron microscopy (FESEM) (Hitachi; S-4300) at an operating voltage of 20 kV and by field emission transmission electron microscopy (FETEM) (JEOL; JEM-2100 F) at an operating voltage of 200 kV by sampling of conventional dimpler and polisher. Clear etch profiles and redepositions on MTJ stacks were confirmed by FETEM and energy dispersive X-ray spectroscopy (EDX) using a spot size of 0.5 nm, and a collection time of 20 s. Optical emission spectroscopy (OES) (Verity; SD1024) was
TiN
35
60000
30
50000
25
40000
20
30000
15
[Ar] [O] [H] [H]/[Ar] [O]/[Ar]
20000 10000
10 5
0 0
Ru CoFeB MgO CoFeB Ru CoFe PtMn
20 nm
TiN
Fig. 2. FETEM micrographs of MTJ stacks etched under 100% H2O. Etch conditions: 800 W ICP power, 300 V dc–bias voltage to substrate, and 0.67 Pa gas pressure.
employed to analyze active species in H2O/Ar and H2O/CH4 plasmas to elucidate the etch mechanism involved in the etching of MTJ stacks. 3. Results and discussion Since MTJ stacks consist of many layers containing metal, magnetic films and tunneling barrier layers, it is very difficult to identify a proper single etch gas. Accordingly, the development of gas combinations is recommended. The etching of MTJ stacks using a CH3OH or CH4/O2 gas mix was recently reported [17,18]. The fundamental concept involved in use of these gases was to employ H, CHx, O or OH components in the plasma. Since then, the use of common H2O vapor to etch MTJ stacks has been proposed. Fig. 1 shows the etch profiles of MTJ stacks etched using H2O/Ar gas containing varying H2O concentrations. The standard etch conditions were an ICP power of 800 W, dc–bias voltage to substrate of 300 V and gas pressure of 0.67 Pa. The profile of MTJ stacks before etching is presented in Fig. 1(a). The thickness of the W/TiN hard mask was 170 nm. The layers of the MTJ stacks and a TiN bottom electrode are shown under the hard mask. The etch profile of MTJ stacks etched Table 1 Atomic percentages obtained by EDX analysis for the points shown in Fig. 2. Element
Pt Mn Co Fe Mg O Ti W Ru Total
Atomic % Point 1 (Fig. 2)
Point 2 (Fig. 2)
17.55 10.77 7.74 5.76 0.36 29.49 23.34 2.46 2.53 100
6.81 4.77 3.30 6.21 1.26 41.00 27.97 1.47 7.21 100
20
30
40
50
60
70
80
90
0 100
% H2O in H2O/Ar gas Fig. 3. OES analysis of plasmas in H2O/Ar gas containing various H2O concentrations.
in pure Ar appeared as a triangle due to the heavy redeposition (Fig. 1(b)), which resulted in imperfect etching of MTJ stacks. As the H2O concentration increased from 10% to 100%, the etch profiles of the MTJ stacks improved with a high degree of anisotropy, leading to clear separation between the MTJ stacks. The MTJ stacks etched under low H2O concentrations were separated between the MTJ stacks, but the space between the MTJ stacks became narrower than 90 nm owing to redeposition on the sidewall of the MTJ stacks. Conversely, the etch profiles of the MTJ stacks etched under high H2O concentrations showed a space of approximately 90 nm and improvement of the high etch slope of approximately 70–80°. These findings indicate that redeposition on the sidewall of the MTJ stacks decreased greatly under high H2O concentrations. Close observation of all etch profiles indicated that etching stopped at a certain layer in the MTJ stacks. For more accurate observation, the MTJ stack etched in 100% H2O was analyzed using FETEM. Fig. 2 shows the FETEM micrograph of MTJ stacks etched in 100% H2O under standard etch conditions (ICP power of 800 W, dc–bias voltage of 300 V, gas pressure of 0.67 Pa). Etching of MTJ stacks was stopped on a MgO thin film (shown as a white layer in Fig. 2) and heavy redeposition occurred on MgO films and the sidewalls of the MTJ stacks. Overall, the results indicated that MgO thin films were rarely etched, even under harsh etching conditions. EDX at points 1 and 2 of Fig. 2 was performed to investigate the redeposited materials. Table 1 shows a variety of elements detected at points 1 and 2. The main elements at point 1 were Ti, O, Pt, Mn and Ru. Ti likely originated from the hard mask because TiN bottom
MTJ stacks Hardmask(HM) MTJ/HM
25
Etch rate (nm/min)
Point1
10
0.4
20
15 0.2
10
5
Etch selectivity (MTJ/HM)
Point 2
70000
Intensity ratio
T.Y. Lee et al. / Thin Solid Films 547 (2013) 146–150
Intensity (arb. units)
148
0.0
0 30
40
50
60
70
80
% CH4 in H2O/CH4 gas Fig. 4. Etch rates of MTJ stacks and hard masks and etch selectivity of MTJ stacks to hard masks using CH4/H2O gas mixtures with various CH4 concentrations. Etch conditions: 800 W ICP power, 300 V dc–bias voltage to substrate, and 0.67 Pa gas pressure.
T.Y. Lee et al. / Thin Solid Films 547 (2013) 146–150
149
(b)
(a)
W/TiN W/TiN
MTJ
MTJ
TiN 100nm
TiN
(c)
(d)
W/TiN W/TiN
MTJ
MTJ TiN TiN
Fig. 5. FESEM micrographs of MTJ stacks etched using H2O/CH4 gas mixtures with different CH4 concentrations: (a) pure H2O, (b) 50% CH4, (c) 67% CH4, and (d) 75% CH4. Etch conditions: 800 W ICP power, 300 V dc–bias voltage to substrate, and 0.67 Pa gas pressure.
electrodes were not yet etched, while O was produced from metal oxides formed during etching in the H2O plasma. Pt, Mn and Ru might have originated from dimpling during the preparation of FETEM because the PtMn layer was not yet etched. As a result, the redeposition at point 1 primarily originated from the hard mask.
TiN Ru CoFeB MgO CoFeB Ru CoFe
Point 2
Point 1
The elements detected at point 2 were Ti, O, Pt, Mn, Co, Fe, Mg and Ru. These metals were redeposited during etching and most of the metals except Ti would be metal oxides in H2O plasma. Therefore, it can be concluded that the etching of MTJ stacks in 100% H2O plasma produced the redeposition and was stopped on a MgO layer. Fig. 3 shows OES analysis of the plasma containing H2O/Ar gas. The [H]/[Ar] and [O]/[Ar] ratios are shown as a function of H2O concentration. The plasma was generated under standard etching conditions as described above and the wavelengths of H, O and Ar were 656.5 nm, 777.2 nm and 738.4 nm, respectively. As the H2O concentration in the H2O/Ar gas increased, the [H]/[Ar] and [O]/[Ar] ratios increased simultaneously. However, the [H]/[Ar] ratio was much larger than the [O]/[Ar] ratio. The increases in hydrogen and oxygen species in the plasma were confirmed by OES and they were shown to play important roles in achieving a high degree of anisotropy and reducing redeposition, respectively. Under high H2O concentrations, a protection layer containing hydrogen was formed on the surface of the MTJ stack, which facilitated etching in the vertical direction, leading to the high etch slope of the
PtMn Table 2 Atomic percentages obtained by EDX analysis for the points shown in Fig. 6. Element
TiN
20nm
Fig. 6. FETEM micrographs of MTJ stacks etched using 75% H2O/CH4 gas. Etch conditions: 800 W ICP power, 300 V dc–bias voltage to substrate, and 0.67 Pa gas pressure.
Pt Mn Co Fe Mg O Ti W Ru Total
Atomic % Point 1 (Fig. 6)
Point 2 (Fig. 6)
2.12 1.31 0.29 0.25 0.67 40.21 51.60 0.06 3.49 100
7.81 5.04 7.22 2.44 1.34 28.19 44.59 0.49 2.88 100
150
T.Y. Lee et al. / Thin Solid Films 547 (2013) 146–150
MTJ stack. The increased O under high H2O concentration formed metal oxide compounds via oxidation during etching, which could be easily removed upon further etching. To solve this problem, a CH4 gas that was applied to etch the MTJ stacks was added to the H2O etching gas. The etch characteristics of MTJ stacks in a CH4 gas have been reported previously [18]. Fig. 4 shows the etch rates of MTJ stacks and W/TiN hard masks, as well as the etch selectivity of the MTJ stack to the W/TiN hard mask using H2O/CH4 gas mixes with varying concentrations of CH4. As the H2O concentration increased from 33.3% to 75% in the H2O/CH4 gas mix, the etch rates of MTJ stacks and hard masks increased slightly. This was because the etch rate of the MTJ stack was faster when CH4 gas was used than when H2O gas was used. However, the etch selectivity of MTJ to the hard mask was maintained at as low as 0.2 because of the very low etch rate of the MTJ stack when the H2O/CH4 gas mix was used. Fig. 5 shows the FESEM micrographs of the MTJ stacks prepared using H2O/CH4 gas mixtures with varying concentrations of CH4. Fig. 5(a) shows the etch profile of the MTJ stack etched using H2O alone. As shown in Fig. 2, there was severe redeposition on the sidewall of the MTJ stacks with narrowed spaces. As the CH4 concentration in H2O/CH4 gas mix increased from 50% to 75%, the etch profiles of the MTJ stacks improved significantly, developing a high degree of anisotropy and the spaces between the MTJ stacks as wide as 90 nm. In addition, the height of the MTJ stacks etched using a 75% CH4/H2O gas mix was reduced to the minimum value observed, but the thickness of the hard mask was sufficient to protect the MTJ stacks. Fig. 6 shows the FETEM micrograph of the MTJ stack etched using the H2O/CH4 gas mix. A good etch profile with an etch slope of approximately 75° was achieved without any redepositions. The W/TiN hard mask was etched significantly because of the very low selectivity of the MTJ stack to hard mask. EDX at points 1 and 2 was carried out to examine the existence of the redeposited materials on the sidewall of the etched MTJ stacks. Table 2 lists the elements detected at points 1 and 2 in Fig. 6. The main elements detected at point 1 were Ti and O, which were obtained from TiN on the bottom electrode and metal oxides via oxidation or from the atmosphere, respectively. Conversely, the elements detected at point 2 consisted of various metals (Pt, Mn, Co, Fe, and Mg) that comprised the MTJ stacks. Among these metals, Pt and Mn are considered to be materials redeposited from the PtMn layer under the MgO thin film. These findings indicate that some redeposition on the sidewall of the etched MTJ stacks occurred. However, the amount of redeposition on the MTJ stacks etched with H2O/CH4 was much smaller than that on MTJ stacks etched using H2O. Therefore, the addition of CH4 to H2O had positive effects on the etch characteristics, including a high degree of anisotropy, less redeposition and a rapid etch rate. 4. Conclusion Etching of MTJ stacks with nanometer-sized hard masks was carried out to investigate the etch characteristics of H2O/Ar gas using an ICPRIE.
As the H2O concentration increased, the redeposition on the sidewalls of the MTJ stacks decreased and the etch slopes improved, especially under 100% H2O. However, FETEM revealed that etching of MTJ stacks in H2O gas was stopped by the MgO layer. Moreover, MgO films were hardly etched in H2O plasma and the etch slope was very slanted. The addition of CH4 gas to H2O gas improved the etch profile of MTJ stacks significantly. As the ratio of CH4 to H2O increased, the redeposition on the sidewalls decreased greatly and a high degree of anisotropy was achieved. Currently, the role of CH4 in the H2O/CH4 gas mix in etching of MTJ stacks with high etch slopes without any redeposition is under investigation.
Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012-0002459). This work was supported by an Inha University Research Grant.
References [1] K. Nordquist, S. Pendharkar, M. Durlam, D. Resnick, S. Tehrani, D. Mancini, T. Zhu, J. Shi, J. Vac. Sci. Technol., B 15 (1997) 2274. [2] S. Tehrani, J.M. Slaughter, E. Chen, M. Durlam, J. Shi, M. DeHerrera, IEEE Trans. Magn. 35 (1999) 2814. [3] S. Tehrani, B. Engel, J.M. Slaughter, E. Chen, M. DeHerrera, M. Durlam, P. Naji, R. Whig, J. Janesky, J. Calder, IEEE Trans. Magn. 36 (2000) 2752. [4] J.G. Zhu, C. Park, Mater. Today 9 (2006) 36. [5] Ricardo C. Sousa, I. Lucian Prejbeanu, C.R. Physique 6 (2005) 1013. [6] D.J. Resnick, S. Pendharkar, K. Kyler, G. Kerszykowski, S. Clemens, H. Tompkins, M. Durlam, S. Tehrani, Microelectron. Eng. 53 (2000) 367. [7] B. Khamsehpour, C.D.W. Wilkinson, J.N. Chapman, Appl. Phys. Lett. 67 (1995) 3194. [8] K.B. Jung, E.S. Lambers, J.R. Childress, S.J. Pearton, J. Vac. Sci. Technol., A 16 (1998) 1697. [9] Y.S. Song, J.W. Kim, C.W. Chung, Thin Solid Films 467 (2004) 172. [10] N. Matsui, K. Mashimo, A. Egami, A. Konishi, O. Okada, T. Tsukada, Vacuum 66 (2002) 479. [11] S.R. Min, H.N. Cho, S.J. Noh, K.W. Kim, S.A. Seo, C.W. Chung, Phys. Status Solidi C 4 (12) (2007) 4416. [12] T. Mukai, N. Oshima, H. Hada, S. Samukawa, J. Vac. Sci. Technol., A 25 (3) (2007) 432. [13] X. Kong, Microelectron. Eng. 85 (2008) 988. [14] K.B. Jung, H. Cho, Y.B. Hahn, D.C. Hays, T. Feng, Y.D. Park, J.R. Childress, S.J. Pearton, Mater. Sci. Eng. B60 (1999) 101. [15] H. Kubota, K. Ueda, Y. Ando, T. Miyazaki, J. Magn. Magn. Mater. 272-276 (2004) E1421. [16] Y. Otani, H. Kubota, A. Fukushima, H. Maehara, T. Osada, S. Yuasa, K. Ando, IEEE Trans. Magn. 43 (2007) 2776. [17] E.H. Kim, T.Y. Lee, C.W. Chung, J. Electrochem. Soc. 159 (3) (2012) H230. [18] E.H. Kim, T.Y. Lee, C.W. Chung, Thin Solid Films 521 (2012) 216.
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Inductively coupled plasma reactive ion etching of magnetic tunnel junction stacks using H2O/CH4 mixture Tea Young Lee, Il Hoon Lee, Chee Won Chung ⁎ Department of Chemical Engineering, Inha University, 253 Yonghyun-Dong, Nam-Ku, Incheon 402-751, Republic of Korea
a r t i c l e
i n f o
Available online 24 April 2013 Keywords: Magnetic random access memory Magnetic tunnel junction stack Inductively coupled plasma reactive ion etching H2O/CH4/Ar
a b s t r a c t Magnetic tunnel junction (MTJ) stacks patterned with hard masks of 90 × 90 nm2 were etched and the etch characteristics were investigated using inductively coupled plasma ion etching (ICPRIE) in a H2O-based gas mix. As the H2O concentration in H2O/Ar mixtures increased, the etch profile of MTJ stacks improved and the redeposition decreased. Field emission transmission electron microscopy revealed that etching of the MTJ stacks in H2O plasma was stopped on a MgO barrier layer on which heavy redeposition occurred; however, the addition of CH4 gas to H2O solved this issue. Specifically, as the CH4 concentration in the H2O/CH4 gas mixture increased, the etch profile became more vertical and the redeposition was reduced considerably. Overall, the etching of MTJ stacks with a high degree of anisotropy without any redeposition was accomplished using a H2O/CH4 gas mixture in an ICPRIE system. © 2013 Elsevier B.V. All rights reserved.
1. Introduction A variety of memory devices have recently been developed to meet the demands of current smart generation. With the growth of smart technologies, the demand for improved memory devices that are non-volatile and have high-density storage and high speed is increasing. To date, dynamic random access memory (DRAM) and a few flash memories have partially satisfied this demand. However, these have many limitations that prevent them from meeting current requests under smart generation. Therefore, many intensive studies have been conducted to develop alternative memory devices including magnetic random access memory (MRAM), resistive random access memory, and phase-change random access memory [1,2]. MRAM is highlighted as a universal device for the next generation of memory because of its high density, rapid access, low power consumption, and non-volatile properties. MRAM devices consist of a complementary metal-oxide semiconductor and magnetic tunneling junction (MTJ) stack that corresponds to a capacitor of DRAM [3,4]. The MTJ stack is a key portion of the MRAM device. MTJ stacks have various ferromagnetic layers that contain Ni, Fe and Co for the induction of large magnetoresistance. These are composed of two ferromagnetic electrodes that have an insulating barrier such as oxides between them. The flow of tunneling current through the electrodes determines the relative magnetization orientation. The change in resistance between the high resistance antiparallel and the low resistance parallel alignment is characterized as tunnel magneto-resistance, which is an essential operating principle in MRAM devices [4–6]. ⁎ Corresponding author. Tel.: +82 32 860 7473; fax: +82 32 872 0959. E-mail address: [email protected] (C.W. Chung). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.04.022
The etching of MTJ stacks is prerequisite for integration of high density MRAM devices. The etching of MTJ stacks with nanometer patterns is known to be tremendously difficult because the metal films in MTJ stacks rarely react with chemically active species in plasma. Conventional etching methods such as ion milling and reactive ion etching have shown some problems including slow etch rate, redeposition on the sidewalls of patterns, poor etch profile and damage during the etching of magnetic materials and MTJ stacks. Inductively coupled plasma reactive ion etching (ICPRIE) has been applied to etch MTJ stacks and magnetic layers using Cl2/Ar, BCl3/Ar, and HBr/Ar gases [7–14]. However, the etch results were not satisfactory and these etching gases were very toxic and corrosive. A few investigations of the etching of MTJ stacks using CO/NH3, CH3OH and CH4/O2/Ar gases have been reported [15–18], and these gases were shown to be non-toxic and to pose no hazards to humans or the environment. Many studies to solve these etching issues have been conducted in attempts to develop etching gases and etch processes. In this study, H2O was used as an etching gas to investigate the etch characteristics of MTJ stacks patterned on a nanometer scale using an ICPRIE system. The etching of MTJ stacks was investigated by varying the H2O concentration in H2O/Ar gas. In addition, the addition of CH4 gas to H2O was proposed to improve the etch results obtained using only H2O/Ar gas. This H2O/CH4 gas mix had positive effects on MTJ etching when compared to the etch results obtained when only H2O/Ar gas was used.
2. Experimental details The MTJ stacks used in this study consisted of various magnetic and metal thin films and tunneling barrier layers. The MTJ structure
T.Y. Lee et al. / Thin Solid Films 547 (2013) 146–150
147
(b)
(a)
W/TiN W/TiN
MTJ
MTJ TiN TiN 100nm
(d)
(c)
W/TiN W/TiN
MTJ
MTJ
TiN
TiN
(e)
(f)
W/TiN MTJ TiN
W/TiN MTJ TiN
Fig. 1. FESEM micrographs of MTJ stacks etched at different H2O concentrations in H2O/Ar gas: (a) before etching, (b) pure Ar, (c) 10% H2O/Ar, and (d) 30% H2O/Ar, (e) 70% H2O/Ar, and (f) 100% H2O.
was W (70)/TiN (100)/Ru (5)/CoFeB (2)/MgO (0.8)/CoFeB (1.5)/Ru (0.8)/CoFe (1.5)/PtMn (15)/TiN (45)/oxide (nm). W and TiN layers were deposited on the tops of MTJ stacks as a hardmask and MgO thin film was employed as a tunneling barrier layer. All films were deposited on silicon-oxide coated Si substrate using a dc-magnetron sputtering method. The hardmask layers, which were composed of W of 70 nm and TiN of 100 nm, were patterned by e-beam lithography using a negative electron (e)-beam resist and then etched by ICPRIE in a Cl2/C2F6/Ar gas mix. After etching of the hardmask, the e-beam resists were stripped off using a stripping solution. Eventually, patterns of a 90 × 90 nm 2 square array that had spaces of 90 nm between each square were prepared on the MTJ stacks. An ICPRIE system (A-Tech System, Korea) was used to etch the MTJ stacks using H2O/Ar gas. The system was composed of a main chamber and a load lock chamber. The samples of MTJ stacks were loaded into the load lock chamber and then inserted into the main chamber. In the main chamber, a susceptor was maintained at the appropriate temperature of 12–15 °C using a cool fluid circulation system and the substrate was then cooled by filling the space between the substrate and susceptor with He gas. The main rf coil, which could supply rf
power at 13.56 MHz, was located at the top of the main chamber to generate a highly dense plasma. The other rf power at 13.56 MHz induced dc–bias voltage to the substrate to adjust the energy of ions in the plasma. The liquid H2O was evaporated in an evaporator at over 100 °C, which was sufficient to produce a flow of H2O vapor. The flow line was heated to prevent the evaporated H2O vapor from condensing in the line. The flow rates of H2O were controlled by an exclusive mass flow controller for H2O. H2O/Ar and H2O/CH4 gas mixtures were employed as the etch gases. The etch characteristics of the MTJ stacks were investigated by varying the H2O concentration in H2O/Ar gas and the CH4 concentration in H2O/CH4 gas. The etch profiles of the MTJ stacks were observed by field emission scanning electron microscopy (FESEM) (Hitachi; S-4300) at an operating voltage of 20 kV and by field emission transmission electron microscopy (FETEM) (JEOL; JEM-2100 F) at an operating voltage of 200 kV by sampling of conventional dimpler and polisher. Clear etch profiles and redepositions on MTJ stacks were confirmed by FETEM and energy dispersive X-ray spectroscopy (EDX) using a spot size of 0.5 nm, and a collection time of 20 s. Optical emission spectroscopy (OES) (Verity; SD1024) was
TiN
35
60000
30
50000
25
40000
20
30000
15
[Ar] [O] [H] [H]/[Ar] [O]/[Ar]
20000 10000
10 5
0 0
Ru CoFeB MgO CoFeB Ru CoFe PtMn
20 nm
TiN
Fig. 2. FETEM micrographs of MTJ stacks etched under 100% H2O. Etch conditions: 800 W ICP power, 300 V dc–bias voltage to substrate, and 0.67 Pa gas pressure.
employed to analyze active species in H2O/Ar and H2O/CH4 plasmas to elucidate the etch mechanism involved in the etching of MTJ stacks. 3. Results and discussion Since MTJ stacks consist of many layers containing metal, magnetic films and tunneling barrier layers, it is very difficult to identify a proper single etch gas. Accordingly, the development of gas combinations is recommended. The etching of MTJ stacks using a CH3OH or CH4/O2 gas mix was recently reported [17,18]. The fundamental concept involved in use of these gases was to employ H, CHx, O or OH components in the plasma. Since then, the use of common H2O vapor to etch MTJ stacks has been proposed. Fig. 1 shows the etch profiles of MTJ stacks etched using H2O/Ar gas containing varying H2O concentrations. The standard etch conditions were an ICP power of 800 W, dc–bias voltage to substrate of 300 V and gas pressure of 0.67 Pa. The profile of MTJ stacks before etching is presented in Fig. 1(a). The thickness of the W/TiN hard mask was 170 nm. The layers of the MTJ stacks and a TiN bottom electrode are shown under the hard mask. The etch profile of MTJ stacks etched Table 1 Atomic percentages obtained by EDX analysis for the points shown in Fig. 2. Element
Pt Mn Co Fe Mg O Ti W Ru Total
Atomic % Point 1 (Fig. 2)
Point 2 (Fig. 2)
17.55 10.77 7.74 5.76 0.36 29.49 23.34 2.46 2.53 100
6.81 4.77 3.30 6.21 1.26 41.00 27.97 1.47 7.21 100
20
30
40
50
60
70
80
90
0 100
% H2O in H2O/Ar gas Fig. 3. OES analysis of plasmas in H2O/Ar gas containing various H2O concentrations.
in pure Ar appeared as a triangle due to the heavy redeposition (Fig. 1(b)), which resulted in imperfect etching of MTJ stacks. As the H2O concentration increased from 10% to 100%, the etch profiles of the MTJ stacks improved with a high degree of anisotropy, leading to clear separation between the MTJ stacks. The MTJ stacks etched under low H2O concentrations were separated between the MTJ stacks, but the space between the MTJ stacks became narrower than 90 nm owing to redeposition on the sidewall of the MTJ stacks. Conversely, the etch profiles of the MTJ stacks etched under high H2O concentrations showed a space of approximately 90 nm and improvement of the high etch slope of approximately 70–80°. These findings indicate that redeposition on the sidewall of the MTJ stacks decreased greatly under high H2O concentrations. Close observation of all etch profiles indicated that etching stopped at a certain layer in the MTJ stacks. For more accurate observation, the MTJ stack etched in 100% H2O was analyzed using FETEM. Fig. 2 shows the FETEM micrograph of MTJ stacks etched in 100% H2O under standard etch conditions (ICP power of 800 W, dc–bias voltage of 300 V, gas pressure of 0.67 Pa). Etching of MTJ stacks was stopped on a MgO thin film (shown as a white layer in Fig. 2) and heavy redeposition occurred on MgO films and the sidewalls of the MTJ stacks. Overall, the results indicated that MgO thin films were rarely etched, even under harsh etching conditions. EDX at points 1 and 2 of Fig. 2 was performed to investigate the redeposited materials. Table 1 shows a variety of elements detected at points 1 and 2. The main elements at point 1 were Ti, O, Pt, Mn and Ru. Ti likely originated from the hard mask because TiN bottom
MTJ stacks Hardmask(HM) MTJ/HM
25
Etch rate (nm/min)
Point1
10
0.4
20
15 0.2
10
5
Etch selectivity (MTJ/HM)
Point 2
70000
Intensity ratio
T.Y. Lee et al. / Thin Solid Films 547 (2013) 146–150
Intensity (arb. units)
148
0.0
0 30
40
50
60
70
80
% CH4 in H2O/CH4 gas Fig. 4. Etch rates of MTJ stacks and hard masks and etch selectivity of MTJ stacks to hard masks using CH4/H2O gas mixtures with various CH4 concentrations. Etch conditions: 800 W ICP power, 300 V dc–bias voltage to substrate, and 0.67 Pa gas pressure.
T.Y. Lee et al. / Thin Solid Films 547 (2013) 146–150
149
(b)
(a)
W/TiN W/TiN
MTJ
MTJ
TiN 100nm
TiN
(c)
(d)
W/TiN W/TiN
MTJ
MTJ TiN TiN
Fig. 5. FESEM micrographs of MTJ stacks etched using H2O/CH4 gas mixtures with different CH4 concentrations: (a) pure H2O, (b) 50% CH4, (c) 67% CH4, and (d) 75% CH4. Etch conditions: 800 W ICP power, 300 V dc–bias voltage to substrate, and 0.67 Pa gas pressure.
electrodes were not yet etched, while O was produced from metal oxides formed during etching in the H2O plasma. Pt, Mn and Ru might have originated from dimpling during the preparation of FETEM because the PtMn layer was not yet etched. As a result, the redeposition at point 1 primarily originated from the hard mask.
TiN Ru CoFeB MgO CoFeB Ru CoFe
Point 2
Point 1
The elements detected at point 2 were Ti, O, Pt, Mn, Co, Fe, Mg and Ru. These metals were redeposited during etching and most of the metals except Ti would be metal oxides in H2O plasma. Therefore, it can be concluded that the etching of MTJ stacks in 100% H2O plasma produced the redeposition and was stopped on a MgO layer. Fig. 3 shows OES analysis of the plasma containing H2O/Ar gas. The [H]/[Ar] and [O]/[Ar] ratios are shown as a function of H2O concentration. The plasma was generated under standard etching conditions as described above and the wavelengths of H, O and Ar were 656.5 nm, 777.2 nm and 738.4 nm, respectively. As the H2O concentration in the H2O/Ar gas increased, the [H]/[Ar] and [O]/[Ar] ratios increased simultaneously. However, the [H]/[Ar] ratio was much larger than the [O]/[Ar] ratio. The increases in hydrogen and oxygen species in the plasma were confirmed by OES and they were shown to play important roles in achieving a high degree of anisotropy and reducing redeposition, respectively. Under high H2O concentrations, a protection layer containing hydrogen was formed on the surface of the MTJ stack, which facilitated etching in the vertical direction, leading to the high etch slope of the
PtMn Table 2 Atomic percentages obtained by EDX analysis for the points shown in Fig. 6. Element
TiN
20nm
Fig. 6. FETEM micrographs of MTJ stacks etched using 75% H2O/CH4 gas. Etch conditions: 800 W ICP power, 300 V dc–bias voltage to substrate, and 0.67 Pa gas pressure.
Pt Mn Co Fe Mg O Ti W Ru Total
Atomic % Point 1 (Fig. 6)
Point 2 (Fig. 6)
2.12 1.31 0.29 0.25 0.67 40.21 51.60 0.06 3.49 100
7.81 5.04 7.22 2.44 1.34 28.19 44.59 0.49 2.88 100
150
T.Y. Lee et al. / Thin Solid Films 547 (2013) 146–150
MTJ stack. The increased O under high H2O concentration formed metal oxide compounds via oxidation during etching, which could be easily removed upon further etching. To solve this problem, a CH4 gas that was applied to etch the MTJ stacks was added to the H2O etching gas. The etch characteristics of MTJ stacks in a CH4 gas have been reported previously [18]. Fig. 4 shows the etch rates of MTJ stacks and W/TiN hard masks, as well as the etch selectivity of the MTJ stack to the W/TiN hard mask using H2O/CH4 gas mixes with varying concentrations of CH4. As the H2O concentration increased from 33.3% to 75% in the H2O/CH4 gas mix, the etch rates of MTJ stacks and hard masks increased slightly. This was because the etch rate of the MTJ stack was faster when CH4 gas was used than when H2O gas was used. However, the etch selectivity of MTJ to the hard mask was maintained at as low as 0.2 because of the very low etch rate of the MTJ stack when the H2O/CH4 gas mix was used. Fig. 5 shows the FESEM micrographs of the MTJ stacks prepared using H2O/CH4 gas mixtures with varying concentrations of CH4. Fig. 5(a) shows the etch profile of the MTJ stack etched using H2O alone. As shown in Fig. 2, there was severe redeposition on the sidewall of the MTJ stacks with narrowed spaces. As the CH4 concentration in H2O/CH4 gas mix increased from 50% to 75%, the etch profiles of the MTJ stacks improved significantly, developing a high degree of anisotropy and the spaces between the MTJ stacks as wide as 90 nm. In addition, the height of the MTJ stacks etched using a 75% CH4/H2O gas mix was reduced to the minimum value observed, but the thickness of the hard mask was sufficient to protect the MTJ stacks. Fig. 6 shows the FETEM micrograph of the MTJ stack etched using the H2O/CH4 gas mix. A good etch profile with an etch slope of approximately 75° was achieved without any redepositions. The W/TiN hard mask was etched significantly because of the very low selectivity of the MTJ stack to hard mask. EDX at points 1 and 2 was carried out to examine the existence of the redeposited materials on the sidewall of the etched MTJ stacks. Table 2 lists the elements detected at points 1 and 2 in Fig. 6. The main elements detected at point 1 were Ti and O, which were obtained from TiN on the bottom electrode and metal oxides via oxidation or from the atmosphere, respectively. Conversely, the elements detected at point 2 consisted of various metals (Pt, Mn, Co, Fe, and Mg) that comprised the MTJ stacks. Among these metals, Pt and Mn are considered to be materials redeposited from the PtMn layer under the MgO thin film. These findings indicate that some redeposition on the sidewall of the etched MTJ stacks occurred. However, the amount of redeposition on the MTJ stacks etched with H2O/CH4 was much smaller than that on MTJ stacks etched using H2O. Therefore, the addition of CH4 to H2O had positive effects on the etch characteristics, including a high degree of anisotropy, less redeposition and a rapid etch rate. 4. Conclusion Etching of MTJ stacks with nanometer-sized hard masks was carried out to investigate the etch characteristics of H2O/Ar gas using an ICPRIE.
As the H2O concentration increased, the redeposition on the sidewalls of the MTJ stacks decreased and the etch slopes improved, especially under 100% H2O. However, FETEM revealed that etching of MTJ stacks in H2O gas was stopped by the MgO layer. Moreover, MgO films were hardly etched in H2O plasma and the etch slope was very slanted. The addition of CH4 gas to H2O gas improved the etch profile of MTJ stacks significantly. As the ratio of CH4 to H2O increased, the redeposition on the sidewalls decreased greatly and a high degree of anisotropy was achieved. Currently, the role of CH4 in the H2O/CH4 gas mix in etching of MTJ stacks with high etch slopes without any redeposition is under investigation.
Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012-0002459). This work was supported by an Inha University Research Grant.
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