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Ar plasma
Microelectronic Engineering 108 (2013) 39–44
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Investigation on etch characteristics of FePt thin films using a H2O/Ar plasma Il Hoon Lee, Tea Young Lee, Chee Won Chung ⇑ Department of Chemical Engineering, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, Republic of Korea
a r t i c l e
i n f o
Article history: Received 25 November 2012 Received in revised form 28 February 2013 Accepted 21 March 2013 Available online 29 March 2013 Keywords: FePt thin films Magnetic tunnel junction Inductively coupled plasma reactive ion etching H2O/Ar gas
a b s t r a c t The etch characteristics of FePt thin films patterned with a TiN hard mask were investigated using inductively coupled plasma reactive ion etching (ICPRIE) in a H2O/Ar plasma. As the H2O concentration increased, the etch rates of the FePt films and TiN hard mask decreased gradually, but the etch profile improved with a high degree of anisotropy without redepositions or etch residues. The improvement of etch profiles was attributed to the formation of a protective layer containing hydrogen species on the sidewall of the patterns and the formation of metal oxides during etching. The optical emission spectroscopy of H2O/Ar plasma revealed an increase of [H]/[Ar] and [O]/[Ar] ratios with increasing H2O concentration. As the ICP rf power and dc-bias voltage increased, and the gas pressure decreased, the etch rates of the FePt films increased and more vertical etch slopes were obtained. X-ray photoelectron spectroscopy of etched FePt films confirmed the existence of Fe oxide compounds formed by the etching. Overall, the results indicated that the etching of FePt films in a H2O/Ar plasma follows a chemically-affected sputtering etching mechanism. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction As advances and progress in IT technology continue, the development of next-generation memory devices with fast access, high density and non-volatility is needed. Among the various memory candidates, magnetic random access memory (MRAM) shows a variety of excellent features including non-volatility, fast access time, unlimited read/write endurance, low operating voltage, and high storage density. Therefore, MRAM is considered a promising alternative memory to current memory devices [1–4]. MRAM consists of a magnetic tunnel junction (MTJ) and complementary metal-oxide semiconductor (CMOS). The MTJ stack is composed of various magnetic materials, metals and a tunneling barrier layer. To achieve high density MRAM, the etching process of the MTJ stack in the MRAM should be developed. However, the etching of magnetic materials and MTJ stacks is generally very difficult because they rarely react with chemically active species in plasma. The etching of magnetic materials was initially carried out using ion milling but this technique was limited by disadvantages such as sidewall redeposition, slanted etch slope and etching damage. Reactive ion beam etching, chemical assisted ion etching (CAIE) and reactive ion etching (RIE) were also applied to etch the magnetic materials, however, these techniques were hampered by slow etch rate, low etch selectivity and a low degree of anisotropy in the etch profile. To overcome these etch problems, high ⇑ Corresponding author. Tel.: +82 32 860 7473; fax: +82 32 872 0959. E-mail address: [email protected] (C.W. Chung). 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.03.125
density plasma etching was conducted to etch the magnetic thin films [5–7]. FePt, FePd and CoPt thin films have recently been applied to MTJ stacks because of their high magneto-crystalline anisotropy and high coercivity [8,9]. Among these magnetic materials, the etching of FePt thin films was carried out to investigate the etch characteristics of the films. The etching of FePt thin films was previously reported using a CH4/O2/NH3 gas mixture [10] and a CH3OH/Ar gas has also been applied to etch FePt thin films [11]. In this study, an inductively coupled plasma reactive ion etching (ICPRIE) of FePt thin films was examined using H2O plasma as a new etching gas. Through this study, the possibility of H2O vapor as a new etching gas was investigated in terms of etch rate, etch selectivity and etch profile. In addition, the etch mechanism of FePt thin films in H2O/Ar gas was elucidated by analyzing the etched surfaces using X-ray photoelectron spectroscopy (XPS) and measuring the active species of the plasmas using optical emission spectroscopy (OES). 2. Experiment details An ICPRIE of FePt thin films patterned with a TiN hard mask was performed in a H2O/Ar plasma. FePt and TiN thin films with a thickness of 100 nm were prepared on Si wafers by direct-current (dc) magnetron sputtering, while 100 nm thick TiN thin films were deposited on FePt films for observation of etch profile. Photolithography was performed using a 1.2 lm-thick photoresist to pattern all of the films. Subsequently, the TiN thin films deposited on the
I.H. Lee et al. / Microelectronic Engineering 108 (2013) 39–44
3. Results and discussion Etching of FePt thin films and a TiN hard mask was performed under H2O/Ar gas containing various concentration of H2O. The etching conditions were as follows: ICP rf power of 800 W, dcbias voltage to substrate of 300 V, and gas pressure of 0.67 Pa. The etch rates of each film and the etch selectivity of FePt film to TiN hard mask are shown in Fig. 1. The etch rate of FePt and TiN films decreased gradually with increasing H2O concentration, which indicates that there was no enhancement in etch rate with increasing H2O concentration. The decrease in the etch
50
200
FePt TiN FePt/TiN
30 100 20 50
0
10
0
20
40
60
80
rate with increasing H2O concentration was attributed to the decrease of the Ar ion flux to the specimen due to the decrease of Ar gas, and the hindrance by species such as –OH or H in the H2O plasma. The etching of FePt thin film is not considered to obey the mechanism of reactive ion etching in H2O/Ar gas. Conversely, as the H2O concentration in H2O/Ar gas increased, the selectivity of FePt increased, showing the highest value of 32 in 20% H2O. As the H2O concentration increased further, the selectivity gradually decreased owing to the abrupt decrease in FePt etch rate. FESEM micrographs of FePt films etched in different H2O concentrations are shown in Fig. 2. The etch profile of FePt films in pure Ar showed a slanted etch slope with a slight redeposition due to the etch mechanism of physical sputtering by Ar ions (Fig. 2(a)). As the H2O concentration increased, the etched sidewall of the patterns became more vertical and smoother without any redeposition or etch residue. The addition of H2O into Ar gas improved the etch profiles of the FePt films with a high degree of anisotropy. For all etch conditions except pure Ar gas, the etch slope, which is defined as a sidewall angle of the etched patterns, was more than 80o. This was attributed to the formation of a
500nm
(d)
(c)
FePt
0
Fig. 1. Etch rate of FePt thin films and TiN hard mask, and etch selectivity of the FePt/TiN in different H2O concentrations in H2O/Ar gas.
TiN FePt
TiN FePt
100
% H2O in H2O/Ar
c (b)
(a)
40
150
Etch selectivity (FePt/TiN)
FePt films were patterned by reactive ion etching in a Cl2/C2F6/Ar gas mix. The photoresist mask was then removed by wet stripping and O2 plasma ashing, which left the patterned TiN hard masks on FePt thin films. The etching of all films was carried out using an ICPRIE system (A-Tech System, Korea), equipped with a main chamber and a loadlock chamber. The substrate in the ICPRIE system was cooled with He gas, which was filled between the substrate and the susceptor. The susceptor was chilled through a cold fluid of a circulator maintaining at 12–15 °C. The main coil, which was connected to a 13.56 MHz rf power supply, was located on the lid in the process chamber to generate high density plasma. A dc-bias voltage induced by rf power at 13.56 MHz was coupled capacitively to the substrate susceptor to control the ion energy at the substrate. H2O/Ar gas was employed as an etch gas and fed into the main chamber. The etch rates and etch profiles of the FePt films and TiN hard mask and the etch selectivity of the FePt films to the TiN hard mask were examined under H2O/Ar gas containing various concentrations of H2O. In addition, the effect of etching parameters on the etch rate and etch profile was also examined by varying the coil rf power, dc-bias voltage to the substrate and gas pressure. The etch rates were measured using a surface profilometer (Tencor P-1). The etch profiles of the films were observed by field emission scanning electron microscopy (FESEM: Hitachi S-4300) at an operating voltage of 20 kV. OES (Verity SD1024) was used to analyze the active species in the H2O/Ar plasma. XPS (ThermoScientific K-Alpha) was conducted to examine the etch products on the etched films, and to elucidate the etch mechanism of FePt thin films in H2O/Ar plasma.
Etch rate (nm/min)
40
TiN FePt
Fig. 2. FESEM micrographs of FePt thin films etched in (a) pure Ar, (b) 40% H2O/Ar, (c) 60% H2O/Ar, and (d) 100% H2O.
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I.H. Lee et al. / Microelectronic Engineering 108 (2013) 39–44
35 [Ar] [O] [O]/[Ar]
(b)
20 15 10
10
Intensity (a.u.)
25
[H]/[Ar] ratio
Intensity (a.u.)
30
8 6 4 2
5 0 20
40
60
[O]/[Ar] ratio
[A [Ar] [H] [H]/[Ar]
(a)
0
80
20
40
60
80
%H2O in H2O/Ar gas
%H2O in H2O/Ar gas
Fig. 3. Optical emission intensity of (a) hydrogen species and (b) oxygen species as a function of H2O concentration; Plasma condition: ICP rf power of 800 W, dc-bias voltage of 300 V, and gas pressure of 0.67 Pa.
25
50
20
40 15 30 10
20
5
10 0
700
900
800
70 60
Etch rate (nm/m min)
Etch rate (nm/m min)
60
30
FePt TiN FePt/TiN
(a)
Etch selectivity (FePt/TiN)
70
H2O to Ar gas. The wavelengths of H, O and Ar used in this measurement were 656.5, 777.2 and 738.4 nm, respectively. As the H2O concentration increased, the [H]/[Ar] ratio increased gradually with the maximum of 30 occurring at 90% H2O. These
TiN FePt
TiN F Pt Fe
500nm
(d)
25 20
40 15 30 10
20
5
10
0
(c)
FePt TiN FePt/TiN
50
0
ICP rf power (W)
(b)
30
(a)
300
200
400
Etch selectivity (FePt/TiN)
protection layer containing hydrogen species on the sidewall of the patterns [12]. Fig. 3 shows the OES analysis of the plasmas containing various H2O concentrations to examine the effect of the addition of
0
dc-bias voltage (V)
(c) ( )
(b)
TiN FePt
TiN FePt
500nm
(d) TiN FePt
Fig. 4. Etch rate of FePt thin films and TiN hard mask, and etch selectivity of the FePt/TiN at different ICP rf powers (a) and FESEM micrographs of FePt thin films etched at (b) 700 W, (c) 800 W and (d) 900 W. Etch conditions: 60% H2O/Ar, dc-bias voltage of 300 V, and gas pressure of 0.67 Pa.
TiN FePt
Fig. 5. Etch rate of FePt thin films and TiN hard mask, and etch selectivity of the FePt/TiN at different dc-bias voltages (a) and FESEM micrographs of FePt thin films etched at (b) 200 V, (c) 300 V and (d) 400 V. Etch conditions: 60% H2O/Ar, ICP rf power of 800 W, and gas pressure of 0.67 Pa.
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I.H. Lee et al. / Microelectronic Engineering 108 (2013) 39–44
findings indicated that the formation of protective layers increased in response to the addition of H2O to Ar gas. This is also responsible for the higher etch slope of the etched FePt films at higher H2O concentration in H2O/Ar gas. As the H2O concentration increased, the [O]/[Ar] ratio also increased with the maximum of 9.5 occurring at 90% H2O. As a result, both [H]/[Ar] and [O]/[Ar] ratios showed very similar tendencies with respect to the H2O concentration. To investigate the effects of etching parameters such as ICP rf power, dc-bias voltage and gas pressure on the etch rate and etch profile, etching of FePt thin films was carried out under various conditions at a constant gas concentration of 60% H2O in H2O/Ar gas. The standard etching condition was 60% H2O/Ar gas, ICP rf power of 800 W, dc-bias voltage of 300 V and gas pressure of 0.67 Pa. Fig. 4(a) shows the etch rates of FePt thin films and the TiN hard mask, as well as the etch selectivity of FePt to TiN during variation of ICP rf power with the other etch conditions fixed. As the ICP rf power increased from 700 to 900 W, the etch rate of the FePt film increased linearly but the etch rate of the TiN hard mask showed little change. Thus, the etch selectivity of FePt to TiN increased significantly. As shown in Fig. 4(b–d), the etch profiles of FePt films improved slightly as the ICP rf power increased from 700 to 900 W. This was attributed to the increased plasma density at high ICP rf power. High density plasma contained more
Etch rate (nm/m min)
60
FePt TiN FePt/TiN
(a)
30 25
50
20
40 30
15
20
10
10
5
0 0.0
1.0
0.5
Etch selectivity (FePt/TiN)
70
0
Gas pressure (Pa)
(b)
(c) TiN FePt
TiN FePt
500nm
(d)
TiN FePt
Fig. 6. Etch rate of FePt thin films and TiN hard mask, and etch selectivity of the FePt/TiN under different gas pressures (a) and FESEM micrographs of FePt thin films etched at (b) 0.13 Pa, (c) 0.67 Pa and (d) 1.33 Pa. Etch conditions: 60% H2O/Ar, ICP rf power of 800 W, and dc-bias voltage of 300 V.
radicals and ions was created at high ICP power, resulting in improvement of the etch profile. The effects of dc-bias voltage to substrate on the etch rate, etch selectivity and etch profiles of the FePt films are shown in Fig. 5. As the dc-bias voltage to substrate increased from 200 to 400 V, the etch rate of FePt increased greatly but the etch rate of TiN films increased only slightly. Therefore, the etch selectivity of FePt film to the TiN hard mask increased. The etch profile of FePt thin films produced using a dc-bias voltage of 400 V became more vertical (Fig. 5(d)) than those produced using a 200 V dc-bias voltage. These results were attributed to the fact that Ar ions have higher bombardment energy under high dc-bias voltage than low dc-bias voltage so that the etch rates of each film increased and the etch profile improved when high dc-bias voltage was used. The coil rf power affects the number of ion and radical, and the dc-bias voltage can control the bombarding energy of ions to the FePt films. Therefore, both etch parameters are key factors in determining the etch rate and etch profile. Fig. 6(a) shows the etch rate and etch selectivity of FePt films and the TiN hard masks under various gas pressures. As the gas pressure increased from 0.13 to 1.33 Pa, the etch rates of FePt films decreased linearly and the etch rate of the TiN hard mask showed a very slight decrease. Due to the variation of the plasma density via the change of bias power and/or the decrease of scattering events in the plasma at low gas pressure, the ion bombardment onto the film surface increased, which resulted in an increased etch rate and better etch profiles under low gas pressure (Fig. 6(b)). Investigation of the effects of parameter variation on the etch rates and etch profiles revealed that the etch rate of FePt films increased and the etch profile improved slightly with increasing ICP rf power, dc-bias voltage, and decreasing gas pressure. XPS analysis of FePt thin films etched under various H2O concentrations was performed. Fig. 7 shows the narrow scans of Fe 2p peaks (a), of O 1s peaks (b) and of Pt 4f peaks (c). Bare FePt thin films without the masks were used as specimens for XPS analysis. All samples were sputtered to remove any contaminants on the film surface before the analysis because the samples were exposed to the atmosphere. The binding energy of the Fe 2p peak for the before-etching specimen was approximately 707.5 eV, which corresponded to the metallic Fe [13]. The main peak of the Fe 2p narrow scan after etching of FePt films in 20% H2O/Ar was almost identical to that of the Fe 2p before-etching specimen (Fig. 7(a)). Conversely, the main peaks of Fe 2p for FePt films etched in 60% H2O/Ar and 100% H2O gases were shifted to approximately 710 eV, which corresponded to the binding energies of Fe oxides such as FeO, Fe2O3 and Fe3O4 [14]. These findings indicated the formation of Fe oxide compounds on the FePt films when etching was conducted using H2O/Ar gas with H2O concentrations exceeding 60%. Fig. 7(b) shows the narrow scans of O 1s for FePt thin films etched using various H2O concentrations. The narrow scan of O 1s for the FePt films before etching showed very wide peak with weak intensity around 529.5 eV. The narrow scan of O 1s for FePt films etched in 20% H2O/Ar gas also showed very weak peak at 529.2 eV. It indicates that there is some possibility of the formation of Fe oxides during etching but the likelihood is low. However, in cases of etching in 60% H2O/Ar and 100% H2O gases, O 1s peaks with high intensity were observed at approximately 530 eV, which corresponded to Fe oxide compounds and indicated the formation of various Fe oxides. On the other hand, the narrow scans of Pt 4f for the before-etching specimen and FePt films etched in 20% H2O/Ar showed metallic Pt peaks with a binding energy of 71.5 eV. The peaks of Pt 4f for FePt films etched in 60% H2O/Ar and 100% H2O gases were very weak and slightly shifted to 72 eV. This might indicate the formation of PtO (72.4 eV) or Pt(OH)2 (72.6 eV) but further investigation is needed to confirm this.
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I.H. Lee et al. / Microelectronic Engineering 108 (2013) 39–44
(b) O 1s
Intensity (arb. unit)
(a) Fe 2p 100% H2O/Ar
100% H2O/Ar
60% H2O/Ar
60% H2O/Ar
20% H2O/Ar
20% H2O/Ar Before etching 735
730
Before etching
725
720
715
710
540
705
Binding Energy (eV)
535
530
525
Binding Energy (eV)
(c) Pt 4f Intensity (arb. unit)
100% H2O/Ar 60% H2O/Ar
20% H2O/Ar Before etching 85
75
80
70
Binding Energy (eV) Fig. 7. XPS narrow scans of (a) Fe 2p, (b) O 1s and (c) Pt 4f for FePt thin films etched using different H2O concentrations. Etch condition: ICP rf power of 800 W, dc-bias voltage of 300 V, and gas pressure of 0.67 Pa.
4. Conclusion ICPRIE of FePt thin films patterned with a TiN hard mask was performed using H2O/Ar gas. As the H2O concentration in the H2O/Ar gas increased, the etch rate of the FePt films and TiN hard mask gradually decreased. The etch selectivity of FePt films to the TiN hard mask decreased gradually as the H2O concentration increased from 20 to 100%. The etch profiles of FePt films improved with a high degree of anisotropy with increasing H2O concentration. This was attributed to the formation of a protective layer containing hydrogen species on the sidewall of the patterns, which led to etching in the vertical direction. OES analysis of the H2O/Ar gas revealed that [H]/[Ar] and [O]/[Ar] ratios increased gradually with increasing H2O concentration. These characteristics are considered to be responsible for the clean and high etch slope obtained at high H2O concentration. As the ICP rf power and dc-bias voltage increased, the etch rate of the FePt films increased and the etch selectivity of FePt films improved slightly. When a high ICP rf power and dc-bias voltage were used, etch profiles with a high degree of anisotropy were obtained without redeposition or etch residue because the plasma density increased or the ions had higher bombardment energy. XPS analysis of the etched FePt films revealed that chemical reactions between FePt film and H2O plasma occurred during etching, which might help obtain a high degree of anisotropy without redepositions in the etching of FePt films. In conclusion, in this
study, the etching of FePt films using H2O/Ar gas follows a chemically-affected sputtering etching mechanism. Acknowledgements This research was supported by a 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. B15 (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] M.C. Gaidis, E.J. O’Sullivan, J.J. Nowak, Y. Lu, S. Kanakasabapathy, P.L. Trouilloud, D.C. Worledge, S. Assefa, K.R. Milkove, G.P. Wright, W.J. Gallagher, IBM J. Res. Dev. 50 (2006) 41. [5] K. Nagahara, T. Mukai, N. Ishiwata, H. Hada, S. Tahara, Jpn. J. Appl. Phys. 42 (2003) L499. [6] R.C. Sousa, P.P. Freitas, IEEE Trans. Magn. 37 (2001) 1973. [7] J.H. Oh, J.H. Park, H.J. Kim, W.C. Jeong, G.H. Koh, G.T. Jeong, I.H. Hwang, T.W. Kim, J.E. Lee, H.J. Kim, S.O. Park, U.I. Jeong, H.S. Jeong, K. Kim, J. Magn. Magn. Mater. 272–276 (2004) 1936. [8] H.S. Wu, X.L. Li, F. Wang, X.H. Xu, Mater. Chem. Phys. 90 (2005) 95.
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[9] B.C. Lim, Y.F. Ding, G.M. Chow, J.S. Chen, J. Magn. Magn. Mater. 303 (2006) 309– 317. [10] K. Ono, M. Takenaka, M. Yamazaki, K. Masuda, S. Cho, T. Wakayama, F. Takano, H. Akinaga, T. Kanazawa, Appl. Surf. Sci. 254 (2008) 7918. [11] E.H. Kim, Y.B. Xiao, S.M. Kong, C.W. Chung, Thin Solid Films 519 (2011) 8223– 8228.
[12] D.Y. Lee, H.N. Cho, C.W. Chung, J. Electrochem. Soc. 155 (2008) 683–687. [13] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray photoelectron spectroscopy, in: J. Chastain, R.C. King (Eds.), Physical Electronics, Inc., 1995. [14] B.J. Tan, K.J. Klabunde, P.M.A. Sherwood, Chem. Mater. 2 (1990) 186–191.
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Investigation on etch characteristics of FePt thin films using a H2O/Ar plasma Il Hoon Lee, Tea Young Lee, Chee Won Chung ⇑ Department of Chemical Engineering, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, Republic of Korea
a r t i c l e
i n f o
Article history: Received 25 November 2012 Received in revised form 28 February 2013 Accepted 21 March 2013 Available online 29 March 2013 Keywords: FePt thin films Magnetic tunnel junction Inductively coupled plasma reactive ion etching H2O/Ar gas
a b s t r a c t The etch characteristics of FePt thin films patterned with a TiN hard mask were investigated using inductively coupled plasma reactive ion etching (ICPRIE) in a H2O/Ar plasma. As the H2O concentration increased, the etch rates of the FePt films and TiN hard mask decreased gradually, but the etch profile improved with a high degree of anisotropy without redepositions or etch residues. The improvement of etch profiles was attributed to the formation of a protective layer containing hydrogen species on the sidewall of the patterns and the formation of metal oxides during etching. The optical emission spectroscopy of H2O/Ar plasma revealed an increase of [H]/[Ar] and [O]/[Ar] ratios with increasing H2O concentration. As the ICP rf power and dc-bias voltage increased, and the gas pressure decreased, the etch rates of the FePt films increased and more vertical etch slopes were obtained. X-ray photoelectron spectroscopy of etched FePt films confirmed the existence of Fe oxide compounds formed by the etching. Overall, the results indicated that the etching of FePt films in a H2O/Ar plasma follows a chemically-affected sputtering etching mechanism. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction As advances and progress in IT technology continue, the development of next-generation memory devices with fast access, high density and non-volatility is needed. Among the various memory candidates, magnetic random access memory (MRAM) shows a variety of excellent features including non-volatility, fast access time, unlimited read/write endurance, low operating voltage, and high storage density. Therefore, MRAM is considered a promising alternative memory to current memory devices [1–4]. MRAM consists of a magnetic tunnel junction (MTJ) and complementary metal-oxide semiconductor (CMOS). The MTJ stack is composed of various magnetic materials, metals and a tunneling barrier layer. To achieve high density MRAM, the etching process of the MTJ stack in the MRAM should be developed. However, the etching of magnetic materials and MTJ stacks is generally very difficult because they rarely react with chemically active species in plasma. The etching of magnetic materials was initially carried out using ion milling but this technique was limited by disadvantages such as sidewall redeposition, slanted etch slope and etching damage. Reactive ion beam etching, chemical assisted ion etching (CAIE) and reactive ion etching (RIE) were also applied to etch the magnetic materials, however, these techniques were hampered by slow etch rate, low etch selectivity and a low degree of anisotropy in the etch profile. To overcome these etch problems, high ⇑ Corresponding author. Tel.: +82 32 860 7473; fax: +82 32 872 0959. E-mail address: [email protected] (C.W. Chung). 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.03.125
density plasma etching was conducted to etch the magnetic thin films [5–7]. FePt, FePd and CoPt thin films have recently been applied to MTJ stacks because of their high magneto-crystalline anisotropy and high coercivity [8,9]. Among these magnetic materials, the etching of FePt thin films was carried out to investigate the etch characteristics of the films. The etching of FePt thin films was previously reported using a CH4/O2/NH3 gas mixture [10] and a CH3OH/Ar gas has also been applied to etch FePt thin films [11]. In this study, an inductively coupled plasma reactive ion etching (ICPRIE) of FePt thin films was examined using H2O plasma as a new etching gas. Through this study, the possibility of H2O vapor as a new etching gas was investigated in terms of etch rate, etch selectivity and etch profile. In addition, the etch mechanism of FePt thin films in H2O/Ar gas was elucidated by analyzing the etched surfaces using X-ray photoelectron spectroscopy (XPS) and measuring the active species of the plasmas using optical emission spectroscopy (OES). 2. Experiment details An ICPRIE of FePt thin films patterned with a TiN hard mask was performed in a H2O/Ar plasma. FePt and TiN thin films with a thickness of 100 nm were prepared on Si wafers by direct-current (dc) magnetron sputtering, while 100 nm thick TiN thin films were deposited on FePt films for observation of etch profile. Photolithography was performed using a 1.2 lm-thick photoresist to pattern all of the films. Subsequently, the TiN thin films deposited on the
I.H. Lee et al. / Microelectronic Engineering 108 (2013) 39–44
3. Results and discussion Etching of FePt thin films and a TiN hard mask was performed under H2O/Ar gas containing various concentration of H2O. The etching conditions were as follows: ICP rf power of 800 W, dcbias voltage to substrate of 300 V, and gas pressure of 0.67 Pa. The etch rates of each film and the etch selectivity of FePt film to TiN hard mask are shown in Fig. 1. The etch rate of FePt and TiN films decreased gradually with increasing H2O concentration, which indicates that there was no enhancement in etch rate with increasing H2O concentration. The decrease in the etch
50
200
FePt TiN FePt/TiN
30 100 20 50
0
10
0
20
40
60
80
rate with increasing H2O concentration was attributed to the decrease of the Ar ion flux to the specimen due to the decrease of Ar gas, and the hindrance by species such as –OH or H in the H2O plasma. The etching of FePt thin film is not considered to obey the mechanism of reactive ion etching in H2O/Ar gas. Conversely, as the H2O concentration in H2O/Ar gas increased, the selectivity of FePt increased, showing the highest value of 32 in 20% H2O. As the H2O concentration increased further, the selectivity gradually decreased owing to the abrupt decrease in FePt etch rate. FESEM micrographs of FePt films etched in different H2O concentrations are shown in Fig. 2. The etch profile of FePt films in pure Ar showed a slanted etch slope with a slight redeposition due to the etch mechanism of physical sputtering by Ar ions (Fig. 2(a)). As the H2O concentration increased, the etched sidewall of the patterns became more vertical and smoother without any redeposition or etch residue. The addition of H2O into Ar gas improved the etch profiles of the FePt films with a high degree of anisotropy. For all etch conditions except pure Ar gas, the etch slope, which is defined as a sidewall angle of the etched patterns, was more than 80o. This was attributed to the formation of a
500nm
(d)
(c)
FePt
0
Fig. 1. Etch rate of FePt thin films and TiN hard mask, and etch selectivity of the FePt/TiN in different H2O concentrations in H2O/Ar gas.
TiN FePt
TiN FePt
100
% H2O in H2O/Ar
c (b)
(a)
40
150
Etch selectivity (FePt/TiN)
FePt films were patterned by reactive ion etching in a Cl2/C2F6/Ar gas mix. The photoresist mask was then removed by wet stripping and O2 plasma ashing, which left the patterned TiN hard masks on FePt thin films. The etching of all films was carried out using an ICPRIE system (A-Tech System, Korea), equipped with a main chamber and a loadlock chamber. The substrate in the ICPRIE system was cooled with He gas, which was filled between the substrate and the susceptor. The susceptor was chilled through a cold fluid of a circulator maintaining at 12–15 °C. The main coil, which was connected to a 13.56 MHz rf power supply, was located on the lid in the process chamber to generate high density plasma. A dc-bias voltage induced by rf power at 13.56 MHz was coupled capacitively to the substrate susceptor to control the ion energy at the substrate. H2O/Ar gas was employed as an etch gas and fed into the main chamber. The etch rates and etch profiles of the FePt films and TiN hard mask and the etch selectivity of the FePt films to the TiN hard mask were examined under H2O/Ar gas containing various concentrations of H2O. In addition, the effect of etching parameters on the etch rate and etch profile was also examined by varying the coil rf power, dc-bias voltage to the substrate and gas pressure. The etch rates were measured using a surface profilometer (Tencor P-1). The etch profiles of the films were observed by field emission scanning electron microscopy (FESEM: Hitachi S-4300) at an operating voltage of 20 kV. OES (Verity SD1024) was used to analyze the active species in the H2O/Ar plasma. XPS (ThermoScientific K-Alpha) was conducted to examine the etch products on the etched films, and to elucidate the etch mechanism of FePt thin films in H2O/Ar plasma.
Etch rate (nm/min)
40
TiN FePt
Fig. 2. FESEM micrographs of FePt thin films etched in (a) pure Ar, (b) 40% H2O/Ar, (c) 60% H2O/Ar, and (d) 100% H2O.
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35 [Ar] [O] [O]/[Ar]
(b)
20 15 10
10
Intensity (a.u.)
25
[H]/[Ar] ratio
Intensity (a.u.)
30
8 6 4 2
5 0 20
40
60
[O]/[Ar] ratio
[A [Ar] [H] [H]/[Ar]
(a)
0
80
20
40
60
80
%H2O in H2O/Ar gas
%H2O in H2O/Ar gas
Fig. 3. Optical emission intensity of (a) hydrogen species and (b) oxygen species as a function of H2O concentration; Plasma condition: ICP rf power of 800 W, dc-bias voltage of 300 V, and gas pressure of 0.67 Pa.
25
50
20
40 15 30 10
20
5
10 0
700
900
800
70 60
Etch rate (nm/m min)
Etch rate (nm/m min)
60
30
FePt TiN FePt/TiN
(a)
Etch selectivity (FePt/TiN)
70
H2O to Ar gas. The wavelengths of H, O and Ar used in this measurement were 656.5, 777.2 and 738.4 nm, respectively. As the H2O concentration increased, the [H]/[Ar] ratio increased gradually with the maximum of 30 occurring at 90% H2O. These
TiN FePt
TiN F Pt Fe
500nm
(d)
25 20
40 15 30 10
20
5
10
0
(c)
FePt TiN FePt/TiN
50
0
ICP rf power (W)
(b)
30
(a)
300
200
400
Etch selectivity (FePt/TiN)
protection layer containing hydrogen species on the sidewall of the patterns [12]. Fig. 3 shows the OES analysis of the plasmas containing various H2O concentrations to examine the effect of the addition of
0
dc-bias voltage (V)
(c) ( )
(b)
TiN FePt
TiN FePt
500nm
(d) TiN FePt
Fig. 4. Etch rate of FePt thin films and TiN hard mask, and etch selectivity of the FePt/TiN at different ICP rf powers (a) and FESEM micrographs of FePt thin films etched at (b) 700 W, (c) 800 W and (d) 900 W. Etch conditions: 60% H2O/Ar, dc-bias voltage of 300 V, and gas pressure of 0.67 Pa.
TiN FePt
Fig. 5. Etch rate of FePt thin films and TiN hard mask, and etch selectivity of the FePt/TiN at different dc-bias voltages (a) and FESEM micrographs of FePt thin films etched at (b) 200 V, (c) 300 V and (d) 400 V. Etch conditions: 60% H2O/Ar, ICP rf power of 800 W, and gas pressure of 0.67 Pa.
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findings indicated that the formation of protective layers increased in response to the addition of H2O to Ar gas. This is also responsible for the higher etch slope of the etched FePt films at higher H2O concentration in H2O/Ar gas. As the H2O concentration increased, the [O]/[Ar] ratio also increased with the maximum of 9.5 occurring at 90% H2O. As a result, both [H]/[Ar] and [O]/[Ar] ratios showed very similar tendencies with respect to the H2O concentration. To investigate the effects of etching parameters such as ICP rf power, dc-bias voltage and gas pressure on the etch rate and etch profile, etching of FePt thin films was carried out under various conditions at a constant gas concentration of 60% H2O in H2O/Ar gas. The standard etching condition was 60% H2O/Ar gas, ICP rf power of 800 W, dc-bias voltage of 300 V and gas pressure of 0.67 Pa. Fig. 4(a) shows the etch rates of FePt thin films and the TiN hard mask, as well as the etch selectivity of FePt to TiN during variation of ICP rf power with the other etch conditions fixed. As the ICP rf power increased from 700 to 900 W, the etch rate of the FePt film increased linearly but the etch rate of the TiN hard mask showed little change. Thus, the etch selectivity of FePt to TiN increased significantly. As shown in Fig. 4(b–d), the etch profiles of FePt films improved slightly as the ICP rf power increased from 700 to 900 W. This was attributed to the increased plasma density at high ICP rf power. High density plasma contained more
Etch rate (nm/m min)
60
FePt TiN FePt/TiN
(a)
30 25
50
20
40 30
15
20
10
10
5
0 0.0
1.0
0.5
Etch selectivity (FePt/TiN)
70
0
Gas pressure (Pa)
(b)
(c) TiN FePt
TiN FePt
500nm
(d)
TiN FePt
Fig. 6. Etch rate of FePt thin films and TiN hard mask, and etch selectivity of the FePt/TiN under different gas pressures (a) and FESEM micrographs of FePt thin films etched at (b) 0.13 Pa, (c) 0.67 Pa and (d) 1.33 Pa. Etch conditions: 60% H2O/Ar, ICP rf power of 800 W, and dc-bias voltage of 300 V.
radicals and ions was created at high ICP power, resulting in improvement of the etch profile. The effects of dc-bias voltage to substrate on the etch rate, etch selectivity and etch profiles of the FePt films are shown in Fig. 5. As the dc-bias voltage to substrate increased from 200 to 400 V, the etch rate of FePt increased greatly but the etch rate of TiN films increased only slightly. Therefore, the etch selectivity of FePt film to the TiN hard mask increased. The etch profile of FePt thin films produced using a dc-bias voltage of 400 V became more vertical (Fig. 5(d)) than those produced using a 200 V dc-bias voltage. These results were attributed to the fact that Ar ions have higher bombardment energy under high dc-bias voltage than low dc-bias voltage so that the etch rates of each film increased and the etch profile improved when high dc-bias voltage was used. The coil rf power affects the number of ion and radical, and the dc-bias voltage can control the bombarding energy of ions to the FePt films. Therefore, both etch parameters are key factors in determining the etch rate and etch profile. Fig. 6(a) shows the etch rate and etch selectivity of FePt films and the TiN hard masks under various gas pressures. As the gas pressure increased from 0.13 to 1.33 Pa, the etch rates of FePt films decreased linearly and the etch rate of the TiN hard mask showed a very slight decrease. Due to the variation of the plasma density via the change of bias power and/or the decrease of scattering events in the plasma at low gas pressure, the ion bombardment onto the film surface increased, which resulted in an increased etch rate and better etch profiles under low gas pressure (Fig. 6(b)). Investigation of the effects of parameter variation on the etch rates and etch profiles revealed that the etch rate of FePt films increased and the etch profile improved slightly with increasing ICP rf power, dc-bias voltage, and decreasing gas pressure. XPS analysis of FePt thin films etched under various H2O concentrations was performed. Fig. 7 shows the narrow scans of Fe 2p peaks (a), of O 1s peaks (b) and of Pt 4f peaks (c). Bare FePt thin films without the masks were used as specimens for XPS analysis. All samples were sputtered to remove any contaminants on the film surface before the analysis because the samples were exposed to the atmosphere. The binding energy of the Fe 2p peak for the before-etching specimen was approximately 707.5 eV, which corresponded to the metallic Fe [13]. The main peak of the Fe 2p narrow scan after etching of FePt films in 20% H2O/Ar was almost identical to that of the Fe 2p before-etching specimen (Fig. 7(a)). Conversely, the main peaks of Fe 2p for FePt films etched in 60% H2O/Ar and 100% H2O gases were shifted to approximately 710 eV, which corresponded to the binding energies of Fe oxides such as FeO, Fe2O3 and Fe3O4 [14]. These findings indicated the formation of Fe oxide compounds on the FePt films when etching was conducted using H2O/Ar gas with H2O concentrations exceeding 60%. Fig. 7(b) shows the narrow scans of O 1s for FePt thin films etched using various H2O concentrations. The narrow scan of O 1s for the FePt films before etching showed very wide peak with weak intensity around 529.5 eV. The narrow scan of O 1s for FePt films etched in 20% H2O/Ar gas also showed very weak peak at 529.2 eV. It indicates that there is some possibility of the formation of Fe oxides during etching but the likelihood is low. However, in cases of etching in 60% H2O/Ar and 100% H2O gases, O 1s peaks with high intensity were observed at approximately 530 eV, which corresponded to Fe oxide compounds and indicated the formation of various Fe oxides. On the other hand, the narrow scans of Pt 4f for the before-etching specimen and FePt films etched in 20% H2O/Ar showed metallic Pt peaks with a binding energy of 71.5 eV. The peaks of Pt 4f for FePt films etched in 60% H2O/Ar and 100% H2O gases were very weak and slightly shifted to 72 eV. This might indicate the formation of PtO (72.4 eV) or Pt(OH)2 (72.6 eV) but further investigation is needed to confirm this.
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(b) O 1s
Intensity (arb. unit)
(a) Fe 2p 100% H2O/Ar
100% H2O/Ar
60% H2O/Ar
60% H2O/Ar
20% H2O/Ar
20% H2O/Ar Before etching 735
730
Before etching
725
720
715
710
540
705
Binding Energy (eV)
535
530
525
Binding Energy (eV)
(c) Pt 4f Intensity (arb. unit)
100% H2O/Ar 60% H2O/Ar
20% H2O/Ar Before etching 85
75
80
70
Binding Energy (eV) Fig. 7. XPS narrow scans of (a) Fe 2p, (b) O 1s and (c) Pt 4f for FePt thin films etched using different H2O concentrations. Etch condition: ICP rf power of 800 W, dc-bias voltage of 300 V, and gas pressure of 0.67 Pa.
4. Conclusion ICPRIE of FePt thin films patterned with a TiN hard mask was performed using H2O/Ar gas. As the H2O concentration in the H2O/Ar gas increased, the etch rate of the FePt films and TiN hard mask gradually decreased. The etch selectivity of FePt films to the TiN hard mask decreased gradually as the H2O concentration increased from 20 to 100%. The etch profiles of FePt films improved with a high degree of anisotropy with increasing H2O concentration. This was attributed to the formation of a protective layer containing hydrogen species on the sidewall of the patterns, which led to etching in the vertical direction. OES analysis of the H2O/Ar gas revealed that [H]/[Ar] and [O]/[Ar] ratios increased gradually with increasing H2O concentration. These characteristics are considered to be responsible for the clean and high etch slope obtained at high H2O concentration. As the ICP rf power and dc-bias voltage increased, the etch rate of the FePt films increased and the etch selectivity of FePt films improved slightly. When a high ICP rf power and dc-bias voltage were used, etch profiles with a high degree of anisotropy were obtained without redeposition or etch residue because the plasma density increased or the ions had higher bombardment energy. XPS analysis of the etched FePt films revealed that chemical reactions between FePt film and H2O plasma occurred during etching, which might help obtain a high degree of anisotropy without redepositions in the etching of FePt films. In conclusion, in this
study, the etching of FePt films using H2O/Ar gas follows a chemically-affected sputtering etching mechanism. Acknowledgements This research was supported by a 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. B15 (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] M.C. Gaidis, E.J. O’Sullivan, J.J. Nowak, Y. Lu, S. Kanakasabapathy, P.L. Trouilloud, D.C. Worledge, S. Assefa, K.R. Milkove, G.P. Wright, W.J. Gallagher, IBM J. Res. Dev. 50 (2006) 41. [5] K. Nagahara, T. Mukai, N. Ishiwata, H. Hada, S. Tahara, Jpn. J. Appl. Phys. 42 (2003) L499. [6] R.C. Sousa, P.P. Freitas, IEEE Trans. Magn. 37 (2001) 1973. [7] J.H. Oh, J.H. Park, H.J. Kim, W.C. Jeong, G.H. Koh, G.T. Jeong, I.H. Hwang, T.W. Kim, J.E. Lee, H.J. Kim, S.O. Park, U.I. Jeong, H.S. Jeong, K. Kim, J. Magn. Magn. Mater. 272–276 (2004) 1936. [8] H.S. Wu, X.L. Li, F. Wang, X.H. Xu, Mater. Chem. Phys. 90 (2005) 95.
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[12] D.Y. Lee, H.N. Cho, C.W. Chung, J. Electrochem. Soc. 155 (2008) 683–687. [13] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray photoelectron spectroscopy, in: J. Chastain, R.C. King (Eds.), Physical Electronics, Inc., 1995. [14] B.J. Tan, K.J. Klabunde, P.M.A. Sherwood, Chem. Mater. 2 (1990) 186–191.