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Ar plasma for phase change memory
Microelectronic Engineering 161 (2016) 69–73
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Research paper
Etching characteristics of phase change material GeTe in inductively coupled BCl3/Ar plasma for phase change memory Yangyang Xia a,b,c, Bo Liu a,b,⁎, Qing Wang a,b,c, Zhonghua Zhang a,b,c, Shasha Li a,b,c, Yonghui Zheng a,b,c, Le Li a,b,c, Sannian Song a,b, Weijun Yin a,b, Dongning Yao a,b, Zhitang Song a,b, Songlin Feng a,b a b c
State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China Shanghai Key Laboratory of Nanofabrication Technology for Memory, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai, 200050, China University of the Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 30 August 2015 Received in revised form 20 March 2016 Accepted 7 April 2016 Available online 18 April 2016 Keywords: GeTe BCl3/Ar Etching ICP Phase change material
a b s t r a c t The dry etching characteristics of phase change material GeTe were investigated in inductively coupled BCl3/Ar plasma. By changing gas ratio, gas pressure, substrate bias power, and inductively coupled plasma (ICP) source power, respectively, various characteristics of GeTe films were investigated about surface roughness, etch rate and profiles. Etching damage was studied by analyzing the X-ray photoelectron spectroscopy results of etched blank GeTe films. We found that the etch rate increases with the increase of BCl3 content and substrate bias power. However, it first increases then decreases with the increase of gas pressure and ICP power. Surface becomes smooth with the increase of gas pressure, but the higher power and substrate bias power lead rougher surface. Little C contamination, oxidation and halogenated layer were remained on the surface during etching process, which can be removed easily by Ar+ sputtering. The stoichiometric ratio of GeTe is stable after being sputtered in tens of seconds on the etched surface, indicating the etching damage is low. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Phase change memory (PCM) has been identified as a promising candidate for the next-generation memory technology because of high speed, reliability, rewritable, good endurance and low power consumption [1–2]. PCM depends on the transition of the electrical resistance between the amorphous and crystalline states by Joule heating, the amorphous state represents the high resistance state (RESET state), and the crystalline state represents low resistance state (SET state). PCM was proposed by S·Ovshinsky in the 1960s [3], which had been developed rapidly in the last few decades. PCM with Ge2Sb2Te5 (GST) was applied in electronics already due to its high-speed phase transformation and excellent performance. Recently, GeTe binary alloy is considered to be one of the candidate materials for the replacement of GST, because it has a much larger resistance for the crystalline and amorphous states, and shows a growth-dominated crystallization mechanism [4]. The fabrication process of PCM is compatible with complementary metal-oxide-semiconductor (CMOS) process. GeTe films can be integrated into PCM cell by etching. The etching characteristics of GeTe make difference on the performance of PCM cell. The purpose of etching ⁎ Corresponding author at: State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China. E-mail addresses: [email protected] (Y. Xia), [email protected] (B. Liu).
http://dx.doi.org/10.1016/j.mee.2016.04.011 0167-9317/© 2016 Elsevier B.V. All rights reserved.
is to achieve a high etch rate, smooth surface, vertical sidewalls, highly anisotropic profiles, especially stable stoichiometric ratio, which meets the application standard of PCM cell fabrication. Dry etching is considered as one of the best ways to fabricate PCM cell with high etch rate and low damage. The ICP is commonly used in investigating the etching characteristics of phase change materials and some other materials recently [5–9]. The ICP system permits generation of high-density plasma at a low pressure as compared with the conventional reactive ion etching (RIE) system. ICP source power and substrate bias power and gas pressure can be controlled independently, allowing plasma density and ion energy can be controlled independently of the chamber pressure [10,11]. High-density plasma etching system, including ICP, is becoming the technique of choice due to higher etch rate and lower damage [11]. Some papers have reported the study of etching characteristics of GST using Cl-base plasma [12–15]. However there doesn't have much information about etching characteristics of GeTe based on Cl-base plasma. In this paper, ICP is applied to investigate etching characteristics of GeTe films by BCl3/Ar plasma. The use of boron chloride (BCl3) has previously demonstrated positive outcomes, including the elimination of water vapor and residual oxygen from the etching chamber and improvement in surface smoothness [16]. Argon has the advantage of relatively complete set of cross and minimal discharge chemistry [17].One expects that BCl3 can play the role to produce volatile etch products of both Ge and Te, and argon can remove the non-volatile products by physical sputtering on the surface during the etching process. We pay much attention on the influence of various
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parameters, such as ICP power, substrate bias power, gas mixing ratio and gas pressure. 2. Experiments The GeTe thin films were deposited on SiO2/Si (100) substrate by cosputtering Ge and Te pure targets at room temperature. The Ge was deposited by direct current (DC) power sputtering at 24 W, while Te was deposited by radio frequency (RF) magnetron power sputtering at 8 W at the same time. The composition of GeTe films were measured by energy dispersive spectroscopy (EDS), and all of samples are amorphous. An ICP etch system (ULVAC, NE-550H) was used in this experiment, which can provide high density plasma by a high power RF source (13.56 MHz) applied on a single coil. Another 400 KHz RF power supply is applied on the bias side. The substrate size is 8-in. (200 mm) diameter and substrates are transferred with a rectilinear cylinder and loaded manually. The silicon carrier is thermally coupled to temperature controlled chuck through a helium backside flow. During the etching process, the shield and chamber temperature were set at 100 °C and 50 °C, respectively. The PR1-1000 A type positive resist was used for pattern. The gas ratio was controlled by flow controller, the total flow rate of BCl3 and Ar was 50 sccm during the process. The blank GeTe films were etched to investigate surface roughness. Atomic force microscopy (AFM) was performed to examine the surface roughness with uncontact operating mode. The size of the scanned area is 5 ∗ 5 μm2, and the lateral and vertical resolution is 0.76 Å respectively. Field-emission scanning electron microscopy (SEM; Hitachi S-4700) was applied to measure the etching depth and profile. The damage and the chemical bonding characteristics of etched GeTe films were measured by X-ray photoelectron spectroscopy (XPS; Kratos AXIS Ultra). To achieve the depth information of etched GeTe film, Ar ion sputtering was applied to remove the surface material. XPS results provide the chemical information on the top surface of Ar ion sputtering area based on various sputtering time. All of the XPS data presented were acquired using Al Kα X rays (1486.6 eV). The XPS narrow scan spectra of all the interesting regions were recorded with pass energy of 40 eV. The spectral resolution of the system at 40 eV pass energy is 0.1 eV, and the size of the scanned ellipse area is 300 ∗ 700 μm2. 3. Results and discussion Fig. 1 (a) shows the etch rate of GeTe films as a function of BCl3/Ar gas mixture ratio, and it was measured at the condition of process gas pressure 3.75 mTorr, substrate bias power 200 W, and ICP power 600 W. It can be seen that etch rate increases with the increase of BCl3 concentration from 20% to 80% in the gas mixture, the etch rate of 940 nm/min is achieved at 80%. In the plasma system, particles could be excited to the energetic states when an electron collides with particle. When we kept the ICP power, the bias power and gas pressure
Fig. 2. SEM of surface and cross-section of GeTe features after etching with BCl3/ (BCl3 + Ar) ratio of: (a) and (f) 20%, (b) and (g) 40%, (c) and (h) 50%, (d) and (i) 60%, (e) and (j) 80%.
Fig. 1. Etch rate of the GeTe as a function (a) BCl3/Ar gas mixture ratio and (b) gas pressure.
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Fig. 3. Etch rate of the GeTe film as a function of (a) substrate bias power and (b) ICP power.
constant, more active BCl+ 2 ions could be excited with the increasing BCl3 flow, so the concentration of active BCl+ 2 increases with increasing BCl3 content in the etching chamber [18–20]. The increase of etch rate may be caused by the increase of density of active BCl+ 2 ions and some other reactive Cl-based radicals, because it follows the principle of ionassistant etching. The chemically reactive Cl-based radicals and Cl atoms react with GeTe forming some volatile etch products and nonvolatile products on the surface. The non-volatile products were taken away by Ar ion physical sputtering, and the volatile products were taken away by vacuum system. The decrease of etch rate from 50% to 60% should be caused by the cooperation of the physical bombardment based on Ar ion and chemical etching based on BCl3 [21–23]. After the 60% concentration of BCl3, the chemical etching was the main process, which probably increases the etch rate due to more volatile products were formed. The effect of gas pressure can be seen from Fig. 1 (b), the etch rate appears to increase linearly when the chamber pressure is below 5.25 mTorr, which might be attributed to the increase in the density of chemically active ions due to the easy dissociation of BCl3 into radicals by electron impact. The etch rate gets to the peak value 876 nm/min at 5.25 mTorr then begins to decrease with the pressure continuously increasing. This phenomenon arises from the lower mean energy of ions due to more collisions in the sheath and the decrease of sheath potential, or the decrease of reactive Cl-based radicals in the plasma [7,9,16,23–26]. Moreover, the angular distribution of ions bombarding the substrate will be broadened, which will leads a less anisotropic ion flux towards to substrate [26]. The etching depths and profiles with various gas mixtures are shown in Fig. 2 (a)–(j) at the condition of substrate bias power 200 W, ICP power 600 W, and gas pressure 3.75 mTorr, respectively. It can be seen that all processes gave a reasonably anisotropic etch profile, and Fig. 2 (c) shows better vertical profile. Etching residues were not observed on the sidewalls of etched film for all etch conditions shown in Fig. 2. It seems that this
gas mixture used in this study, increasing BCl3 and decreasing Ar concentration, and vice versa could compensate etch other effect and this might explain the effect observed [7,8,23,24]. When the BCl3 concentration is 60%, the etched surface is extremely rough, which may be caused by non-volatile products on the surface. Fig. 3 shows the effects of substrate bias power and ICP power on etch rate and surface roughness. Fig. 3 (a) shows both etch rate and roughness increase with the increasing substrate bias power. When the substrate bias power becomes higher, the energy of ion flux will increases more, which promotes physical bombardment on the surface. However, the density of ion is probably little changed [10,18]. The rougher surface should be attributed to the increase of bombarding energy of energetic Ar+ [7–9]. It is expected that the surface sputtering may results in an increase in surface roughness [16]. Fig. 3 (b) shows the influence of ICP power on etch rate and surface roughness at condition of substrate bias power 200 W, gas pressure 3.75 mTorr, and BCl3/Ar = 25/25. It is found that the etch rate increases monotonically with the increase of power up to 600 W, and reaches the maximum value 606 nm/min at 600 W, then decreases with the increasing power from 606 nm/min to 358 nm/min. When the power is less than 600 W, more chemically reactive ion species were excited to the substrate due to more collisions in the etching chamber and the increase of the average electron temperature related to the increasing ICP power [27]. The initial increase in the etch rate with the increasing power is a result of the increase rate of chemical reactions on the etched surface, which may be caused by the increase in the density of the reactive Cl-based radicals [10,28–29]. When the power exceeds 600 W, the etch rate appears to decrease. It is known that the higher ICP power results in higher plasma density and thinner sheaths and lower sheath potential for the same bias power. For the higher ICP power, the ions have a shorter transit time across the sheath, resulting in a broader IED [25–26,30–31]. In addition, the average ion energy in the vicinity
Fig. 4. XPS spectra of (a)Te3d, (b)Ge2p and (c) Ge3d before and after Ar ion sputtering.
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Fig. 5. XPS spectra of (a) C1s, (b) Cl2p and (c) O1s after and before Ar ion sputtering.
of substrate may decrease because that a higher ion flux at the sheath reduces the sheath potential for the same bias power, gas mixture and gas pressure [26]. The decrease in etch rate of GeTe films is mainly due to the lower kinetic ion mean bombardment energy, which reduces the sputter desorption of etch products on the surface [32,33]. The increase in the surface RMS roughness up to 600 W is mainly caused by more non-volatile by-products on the surface. The insufficient physical desorption may mainly be responsible for the continuous increase in the surface RMS roughness from 600 W to 700 W. Fig. 4 shows the XPS spectra of each component of the GeTe films after etching with these parameters: BCl3/(BCl3 + Ar), ICP power, substrate bias power and gas pressure is 50%, 600 W, 200 W, and 3.75 mTorr, respectively. The sample is exposed to air prior to the analysis oxidation after taking it out from the etching chamber. As shown in Fig. 4 (a), the binding energy peak of Te3d was located at 573.1 eV and 576.7 eV on the surface. The peak at 573.1 eV is considered to be the metallic bonding of Te-Te and 572.5 eV for Te-Ge bonding, and the peak at 576.7 eV was concerned with Te-Clx and Te-Ox [7,8]. However, it is hard to distinguish the bonds of Te-Ox and Te-Clx because their binding energy is very close. It can be seen that 576.7 eV peak disappear after 30s sputtering by Ar ion, which indicates that some non-volatile Te-Clx and Te-Ox were left on the surface. In terms of Ge, the peak of Ge2p and Ge3d were observed at 1218.8 eV and 30.0 eV respectively, which are related to metallic bonding Ge-Te. However, there is another peak at 33.2 eV as shown in Fig. 4(c), which is related to the Ge-Clx and Ge-Ox. It is also hard to distinguish the bonds of Ge-Ox and Ge-Clx because their binding energy is very close. They were removed after
30 s Ar+ sputtering, which indicates the etch residues of Ge-Clx and Ge-Ox were also remained on surface. It is apparent that halogenated residues were accumulated on the etched surface,which are terrible for surface roughness and performance of device. To analyze the residues, XPS spectra of C1s, O1s and Cl2p were also performed as shown in Fig. 5. The peak of C1s at 284.8 eV was considered to be contamination on the surface from C element in the air and disappeared after 30 s Ar ion sputtering. In terms of Cl2p, the spectra peak was found at 200.3 eV, which might be Te-Clx [8,9]. It suggests that the halogenated layer was thin because this peak was removed after 30 s Ar ion sputtering. However, the oxidation layer was a little thicker than the halogenated layer as indicated by increased time necessary to remove oxidation layer as shown in Fig. 5 (c). The contamination of O element is also mainly from the air. The total Ar ion milling depth is estimated about 10 ± 5 nm. The atomic percent change of Ge and Te elements as a function of sputtering time was shown in Fig. 6. It is obvious that the concentration of Ge is much less than the concentration of Te on the etched surface, the concentration of Ge gets close to Te with the increase of sputtering time by Ar ion. It suggests that Cl ions can easily form volatile products with Ge as shown in the Table 1. The Ge atoms might migrate to the surface form Ge-Clx, which can be easily taken away from the chamber. However, much non-volatile Te-Clx is left on the etched surface. 4. Conclusions In this paper, ICP are applied to study etching characteristics of GeTe films. To investigate the etch characteristics and mechanism of GeTe, various etching parameters were adopted to the etching process, for instance ICP power, substrate bias power, chamber gas pressure and gas mixture ratio. The etch rate increases with the increase of substrate bias power and BCl3 concentration, and the surface roughness also increase with the increase of substrate bias power. However, it is different from the trend of substrate bias power and BCl3 concentration, as power and gas pressure increase, the etch rate originally increases and then decreases. All of the etching parameters can affect the density and energy of ions, which can be used to control the etch process. These results of XPS reveal that some by-products accumulated on the etched surface and particularly the Te-Clx was remained. These residues were taken
Table 1 Boiling point of the Ge-Clx and Te-Clx [34].
Fig. 6. Composition change of the GeTe film surface as a function of sputtering time.
Material
Bonding
Boiling point(°C)
Ge Ge Te Te
GeCl4 GeCl2 TeCl2 [TeCl4]4
86.6 ↑ 328 387
Y. Xia et al. / Microelectronic Engineering 161 (2016) 69–73
away easily by Ar+ ion sputtering, which has little damage to the films. After optimizing the etching parameters, we can achieve high etch rate, smooth surface, vertical sidewalls, highly anisotropic profiles, stable stoichiometric ratio at the condition of BCl 3/Ar = 25/25 sccm, ICP power 600 W, substrate bias power 200 W and gas pressure 3.75 mTorr.
Acknowledgements This work was supported by the “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDA09020402), National Key Basic Research Program of China (2013CBA01900, 2010CB934300, 2011CBA00607, 2011CB932804), National Integrate Circuit Research Program of China (2009ZX02023-003), National Natural Science Foundation of China (61176122, 61106001, 61261160500, 61376006, 61401444), Science and Technology Council of Shanghai (12nm0503701, 13DZ2295700, 12QA1403900, 13ZR1447200, 14ZR1447500). References [1] G. Atwood, Phase change materials for electronic memories, Science 321 (2008) 210–211. [2] M. Wuttig, N. Yamada, Phase-change materials for rewriteable data storage, Nat. Mater. 6 (2007) 824–832. [3] S.R. Ovshinsky, Reversible electrical switching phenomena in disordered structures, Phys. Rev.Lett. 21 (1968) 1450–1453. [4] C. Peng, F. Rao, L.C. Wu, Z.T. Song, Y.·.F. Gu, D. Zhou, H.J. Song, P.X. Yang, J.H. Chu, Homogeneous phase W-Ge-Te material with improved overall phase-change properties for future nonvolatile memory, Acta.Mater. 74 (2014) 49–57. [5] Y.·.H. Joo, J.C. Woo, C.·.I. Kim, Surface reaction effects on dry etching of IGZO thin films in N2/BCl3/Ar plasma, Microelectron. Eng. 112 (2013) 74–79. [6] K.·.H. Kwon, Alexander Efremov, S.J. Yun, Iwoo Chun, K. Kim, Dry etching characteristics of Mo and Al2O3 films in O2/Cl2/Ar inductively coupled plasmas, Thin Solid Films 552 (2014) 105–110. [7] J. Zhou, Y. Chen, W.L. Zhou, X.S. Miao, Z. Yang, N.·.N. Yu, H. Liu, T. Lan, J.B. Yan, Inductively coupled plasma etching for phase-change material with superlattic-like structure in phase change memory device, Appl. Surf. Sci. 280 (2013) 862–867. [8] Z.H. Zhang, S.·.S. Song, Z.T. song, Y. Cheng, C. Peng, L. Zhang, D.C. Cao, X.H. Guo, L.C. Wu, B. Liu, Characteristics and mechanism of Al1.3Sb3Te etched by Cl2/BCl3 inductively coupled plasmas, Microelectron. Eng. 115 (2015) 51–54. [9] Z.H. Zhang, S.·.S. Song, Z.T. song, Y. Cheng, M. Zhu, X.Y. Li, Y.Q. Zhu, X.H. Guo, W.J. Yin, L.C. Wu, B. Liu, S.L. Feng, D. Zhou, Etching of new phase change material Ti0.5Sb2Te3 by Cl2/Ar and CF4/Ar inductively coupled plasmas, Appl. Surf. Sci. 311 (2014) 68–73. [10] Kim Han-Ki, J.W. Bae, T.–.K. Kim, K.–.K. Kim, T.–.Y. Seong, I. Adesida, Inductively coupled plasma reactive ion etching of ZnO using BCl3-based plasmas, J.Vac.Sci.Technol. B 21 (2003) 1273–1277. [11] R.J. Shul, G.B. McClellan, R.D. Briggs, D.J. Rieger, S.J. Pearton, C.R. Abernathy, J.W. Lee, C. Constantine, C. Barratt, High-density plasma etching of compound semiconductors, J. Vac. Sci. Technol. A 15 (1997) 633–637. [12] K.·.Y. Yang, S.·.H. Hong, D.K. Kim, B.k Cheong, H. Lee, Patterning of Ge2Sb2Te5 phase change material using UV nano-imprint lithography, Microelectron.Eng. 84 (2007) 21–24.
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Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Research paper
Etching characteristics of phase change material GeTe in inductively coupled BCl3/Ar plasma for phase change memory Yangyang Xia a,b,c, Bo Liu a,b,⁎, Qing Wang a,b,c, Zhonghua Zhang a,b,c, Shasha Li a,b,c, Yonghui Zheng a,b,c, Le Li a,b,c, Sannian Song a,b, Weijun Yin a,b, Dongning Yao a,b, Zhitang Song a,b, Songlin Feng a,b a b c
State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China Shanghai Key Laboratory of Nanofabrication Technology for Memory, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai, 200050, China University of the Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 30 August 2015 Received in revised form 20 March 2016 Accepted 7 April 2016 Available online 18 April 2016 Keywords: GeTe BCl3/Ar Etching ICP Phase change material
a b s t r a c t The dry etching characteristics of phase change material GeTe were investigated in inductively coupled BCl3/Ar plasma. By changing gas ratio, gas pressure, substrate bias power, and inductively coupled plasma (ICP) source power, respectively, various characteristics of GeTe films were investigated about surface roughness, etch rate and profiles. Etching damage was studied by analyzing the X-ray photoelectron spectroscopy results of etched blank GeTe films. We found that the etch rate increases with the increase of BCl3 content and substrate bias power. However, it first increases then decreases with the increase of gas pressure and ICP power. Surface becomes smooth with the increase of gas pressure, but the higher power and substrate bias power lead rougher surface. Little C contamination, oxidation and halogenated layer were remained on the surface during etching process, which can be removed easily by Ar+ sputtering. The stoichiometric ratio of GeTe is stable after being sputtered in tens of seconds on the etched surface, indicating the etching damage is low. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Phase change memory (PCM) has been identified as a promising candidate for the next-generation memory technology because of high speed, reliability, rewritable, good endurance and low power consumption [1–2]. PCM depends on the transition of the electrical resistance between the amorphous and crystalline states by Joule heating, the amorphous state represents the high resistance state (RESET state), and the crystalline state represents low resistance state (SET state). PCM was proposed by S·Ovshinsky in the 1960s [3], which had been developed rapidly in the last few decades. PCM with Ge2Sb2Te5 (GST) was applied in electronics already due to its high-speed phase transformation and excellent performance. Recently, GeTe binary alloy is considered to be one of the candidate materials for the replacement of GST, because it has a much larger resistance for the crystalline and amorphous states, and shows a growth-dominated crystallization mechanism [4]. The fabrication process of PCM is compatible with complementary metal-oxide-semiconductor (CMOS) process. GeTe films can be integrated into PCM cell by etching. The etching characteristics of GeTe make difference on the performance of PCM cell. The purpose of etching ⁎ Corresponding author at: State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China. E-mail addresses: [email protected] (Y. Xia), [email protected] (B. Liu).
http://dx.doi.org/10.1016/j.mee.2016.04.011 0167-9317/© 2016 Elsevier B.V. All rights reserved.
is to achieve a high etch rate, smooth surface, vertical sidewalls, highly anisotropic profiles, especially stable stoichiometric ratio, which meets the application standard of PCM cell fabrication. Dry etching is considered as one of the best ways to fabricate PCM cell with high etch rate and low damage. The ICP is commonly used in investigating the etching characteristics of phase change materials and some other materials recently [5–9]. The ICP system permits generation of high-density plasma at a low pressure as compared with the conventional reactive ion etching (RIE) system. ICP source power and substrate bias power and gas pressure can be controlled independently, allowing plasma density and ion energy can be controlled independently of the chamber pressure [10,11]. High-density plasma etching system, including ICP, is becoming the technique of choice due to higher etch rate and lower damage [11]. Some papers have reported the study of etching characteristics of GST using Cl-base plasma [12–15]. However there doesn't have much information about etching characteristics of GeTe based on Cl-base plasma. In this paper, ICP is applied to investigate etching characteristics of GeTe films by BCl3/Ar plasma. The use of boron chloride (BCl3) has previously demonstrated positive outcomes, including the elimination of water vapor and residual oxygen from the etching chamber and improvement in surface smoothness [16]. Argon has the advantage of relatively complete set of cross and minimal discharge chemistry [17].One expects that BCl3 can play the role to produce volatile etch products of both Ge and Te, and argon can remove the non-volatile products by physical sputtering on the surface during the etching process. We pay much attention on the influence of various
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parameters, such as ICP power, substrate bias power, gas mixing ratio and gas pressure. 2. Experiments The GeTe thin films were deposited on SiO2/Si (100) substrate by cosputtering Ge and Te pure targets at room temperature. The Ge was deposited by direct current (DC) power sputtering at 24 W, while Te was deposited by radio frequency (RF) magnetron power sputtering at 8 W at the same time. The composition of GeTe films were measured by energy dispersive spectroscopy (EDS), and all of samples are amorphous. An ICP etch system (ULVAC, NE-550H) was used in this experiment, which can provide high density plasma by a high power RF source (13.56 MHz) applied on a single coil. Another 400 KHz RF power supply is applied on the bias side. The substrate size is 8-in. (200 mm) diameter and substrates are transferred with a rectilinear cylinder and loaded manually. The silicon carrier is thermally coupled to temperature controlled chuck through a helium backside flow. During the etching process, the shield and chamber temperature were set at 100 °C and 50 °C, respectively. The PR1-1000 A type positive resist was used for pattern. The gas ratio was controlled by flow controller, the total flow rate of BCl3 and Ar was 50 sccm during the process. The blank GeTe films were etched to investigate surface roughness. Atomic force microscopy (AFM) was performed to examine the surface roughness with uncontact operating mode. The size of the scanned area is 5 ∗ 5 μm2, and the lateral and vertical resolution is 0.76 Å respectively. Field-emission scanning electron microscopy (SEM; Hitachi S-4700) was applied to measure the etching depth and profile. The damage and the chemical bonding characteristics of etched GeTe films were measured by X-ray photoelectron spectroscopy (XPS; Kratos AXIS Ultra). To achieve the depth information of etched GeTe film, Ar ion sputtering was applied to remove the surface material. XPS results provide the chemical information on the top surface of Ar ion sputtering area based on various sputtering time. All of the XPS data presented were acquired using Al Kα X rays (1486.6 eV). The XPS narrow scan spectra of all the interesting regions were recorded with pass energy of 40 eV. The spectral resolution of the system at 40 eV pass energy is 0.1 eV, and the size of the scanned ellipse area is 300 ∗ 700 μm2. 3. Results and discussion Fig. 1 (a) shows the etch rate of GeTe films as a function of BCl3/Ar gas mixture ratio, and it was measured at the condition of process gas pressure 3.75 mTorr, substrate bias power 200 W, and ICP power 600 W. It can be seen that etch rate increases with the increase of BCl3 concentration from 20% to 80% in the gas mixture, the etch rate of 940 nm/min is achieved at 80%. In the plasma system, particles could be excited to the energetic states when an electron collides with particle. When we kept the ICP power, the bias power and gas pressure
Fig. 2. SEM of surface and cross-section of GeTe features after etching with BCl3/ (BCl3 + Ar) ratio of: (a) and (f) 20%, (b) and (g) 40%, (c) and (h) 50%, (d) and (i) 60%, (e) and (j) 80%.
Fig. 1. Etch rate of the GeTe as a function (a) BCl3/Ar gas mixture ratio and (b) gas pressure.
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Fig. 3. Etch rate of the GeTe film as a function of (a) substrate bias power and (b) ICP power.
constant, more active BCl+ 2 ions could be excited with the increasing BCl3 flow, so the concentration of active BCl+ 2 increases with increasing BCl3 content in the etching chamber [18–20]. The increase of etch rate may be caused by the increase of density of active BCl+ 2 ions and some other reactive Cl-based radicals, because it follows the principle of ionassistant etching. The chemically reactive Cl-based radicals and Cl atoms react with GeTe forming some volatile etch products and nonvolatile products on the surface. The non-volatile products were taken away by Ar ion physical sputtering, and the volatile products were taken away by vacuum system. The decrease of etch rate from 50% to 60% should be caused by the cooperation of the physical bombardment based on Ar ion and chemical etching based on BCl3 [21–23]. After the 60% concentration of BCl3, the chemical etching was the main process, which probably increases the etch rate due to more volatile products were formed. The effect of gas pressure can be seen from Fig. 1 (b), the etch rate appears to increase linearly when the chamber pressure is below 5.25 mTorr, which might be attributed to the increase in the density of chemically active ions due to the easy dissociation of BCl3 into radicals by electron impact. The etch rate gets to the peak value 876 nm/min at 5.25 mTorr then begins to decrease with the pressure continuously increasing. This phenomenon arises from the lower mean energy of ions due to more collisions in the sheath and the decrease of sheath potential, or the decrease of reactive Cl-based radicals in the plasma [7,9,16,23–26]. Moreover, the angular distribution of ions bombarding the substrate will be broadened, which will leads a less anisotropic ion flux towards to substrate [26]. The etching depths and profiles with various gas mixtures are shown in Fig. 2 (a)–(j) at the condition of substrate bias power 200 W, ICP power 600 W, and gas pressure 3.75 mTorr, respectively. It can be seen that all processes gave a reasonably anisotropic etch profile, and Fig. 2 (c) shows better vertical profile. Etching residues were not observed on the sidewalls of etched film for all etch conditions shown in Fig. 2. It seems that this
gas mixture used in this study, increasing BCl3 and decreasing Ar concentration, and vice versa could compensate etch other effect and this might explain the effect observed [7,8,23,24]. When the BCl3 concentration is 60%, the etched surface is extremely rough, which may be caused by non-volatile products on the surface. Fig. 3 shows the effects of substrate bias power and ICP power on etch rate and surface roughness. Fig. 3 (a) shows both etch rate and roughness increase with the increasing substrate bias power. When the substrate bias power becomes higher, the energy of ion flux will increases more, which promotes physical bombardment on the surface. However, the density of ion is probably little changed [10,18]. The rougher surface should be attributed to the increase of bombarding energy of energetic Ar+ [7–9]. It is expected that the surface sputtering may results in an increase in surface roughness [16]. Fig. 3 (b) shows the influence of ICP power on etch rate and surface roughness at condition of substrate bias power 200 W, gas pressure 3.75 mTorr, and BCl3/Ar = 25/25. It is found that the etch rate increases monotonically with the increase of power up to 600 W, and reaches the maximum value 606 nm/min at 600 W, then decreases with the increasing power from 606 nm/min to 358 nm/min. When the power is less than 600 W, more chemically reactive ion species were excited to the substrate due to more collisions in the etching chamber and the increase of the average electron temperature related to the increasing ICP power [27]. The initial increase in the etch rate with the increasing power is a result of the increase rate of chemical reactions on the etched surface, which may be caused by the increase in the density of the reactive Cl-based radicals [10,28–29]. When the power exceeds 600 W, the etch rate appears to decrease. It is known that the higher ICP power results in higher plasma density and thinner sheaths and lower sheath potential for the same bias power. For the higher ICP power, the ions have a shorter transit time across the sheath, resulting in a broader IED [25–26,30–31]. In addition, the average ion energy in the vicinity
Fig. 4. XPS spectra of (a)Te3d, (b)Ge2p and (c) Ge3d before and after Ar ion sputtering.
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Fig. 5. XPS spectra of (a) C1s, (b) Cl2p and (c) O1s after and before Ar ion sputtering.
of substrate may decrease because that a higher ion flux at the sheath reduces the sheath potential for the same bias power, gas mixture and gas pressure [26]. The decrease in etch rate of GeTe films is mainly due to the lower kinetic ion mean bombardment energy, which reduces the sputter desorption of etch products on the surface [32,33]. The increase in the surface RMS roughness up to 600 W is mainly caused by more non-volatile by-products on the surface. The insufficient physical desorption may mainly be responsible for the continuous increase in the surface RMS roughness from 600 W to 700 W. Fig. 4 shows the XPS spectra of each component of the GeTe films after etching with these parameters: BCl3/(BCl3 + Ar), ICP power, substrate bias power and gas pressure is 50%, 600 W, 200 W, and 3.75 mTorr, respectively. The sample is exposed to air prior to the analysis oxidation after taking it out from the etching chamber. As shown in Fig. 4 (a), the binding energy peak of Te3d was located at 573.1 eV and 576.7 eV on the surface. The peak at 573.1 eV is considered to be the metallic bonding of Te-Te and 572.5 eV for Te-Ge bonding, and the peak at 576.7 eV was concerned with Te-Clx and Te-Ox [7,8]. However, it is hard to distinguish the bonds of Te-Ox and Te-Clx because their binding energy is very close. It can be seen that 576.7 eV peak disappear after 30s sputtering by Ar ion, which indicates that some non-volatile Te-Clx and Te-Ox were left on the surface. In terms of Ge, the peak of Ge2p and Ge3d were observed at 1218.8 eV and 30.0 eV respectively, which are related to metallic bonding Ge-Te. However, there is another peak at 33.2 eV as shown in Fig. 4(c), which is related to the Ge-Clx and Ge-Ox. It is also hard to distinguish the bonds of Ge-Ox and Ge-Clx because their binding energy is very close. They were removed after
30 s Ar+ sputtering, which indicates the etch residues of Ge-Clx and Ge-Ox were also remained on surface. It is apparent that halogenated residues were accumulated on the etched surface,which are terrible for surface roughness and performance of device. To analyze the residues, XPS spectra of C1s, O1s and Cl2p were also performed as shown in Fig. 5. The peak of C1s at 284.8 eV was considered to be contamination on the surface from C element in the air and disappeared after 30 s Ar ion sputtering. In terms of Cl2p, the spectra peak was found at 200.3 eV, which might be Te-Clx [8,9]. It suggests that the halogenated layer was thin because this peak was removed after 30 s Ar ion sputtering. However, the oxidation layer was a little thicker than the halogenated layer as indicated by increased time necessary to remove oxidation layer as shown in Fig. 5 (c). The contamination of O element is also mainly from the air. The total Ar ion milling depth is estimated about 10 ± 5 nm. The atomic percent change of Ge and Te elements as a function of sputtering time was shown in Fig. 6. It is obvious that the concentration of Ge is much less than the concentration of Te on the etched surface, the concentration of Ge gets close to Te with the increase of sputtering time by Ar ion. It suggests that Cl ions can easily form volatile products with Ge as shown in the Table 1. The Ge atoms might migrate to the surface form Ge-Clx, which can be easily taken away from the chamber. However, much non-volatile Te-Clx is left on the etched surface. 4. Conclusions In this paper, ICP are applied to study etching characteristics of GeTe films. To investigate the etch characteristics and mechanism of GeTe, various etching parameters were adopted to the etching process, for instance ICP power, substrate bias power, chamber gas pressure and gas mixture ratio. The etch rate increases with the increase of substrate bias power and BCl3 concentration, and the surface roughness also increase with the increase of substrate bias power. However, it is different from the trend of substrate bias power and BCl3 concentration, as power and gas pressure increase, the etch rate originally increases and then decreases. All of the etching parameters can affect the density and energy of ions, which can be used to control the etch process. These results of XPS reveal that some by-products accumulated on the etched surface and particularly the Te-Clx was remained. These residues were taken
Table 1 Boiling point of the Ge-Clx and Te-Clx [34].
Fig. 6. Composition change of the GeTe film surface as a function of sputtering time.
Material
Bonding
Boiling point(°C)
Ge Ge Te Te
GeCl4 GeCl2 TeCl2 [TeCl4]4
86.6 ↑ 328 387
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away easily by Ar+ ion sputtering, which has little damage to the films. After optimizing the etching parameters, we can achieve high etch rate, smooth surface, vertical sidewalls, highly anisotropic profiles, stable stoichiometric ratio at the condition of BCl 3/Ar = 25/25 sccm, ICP power 600 W, substrate bias power 200 W and gas pressure 3.75 mTorr.
Acknowledgements This work was supported by the “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDA09020402), National Key Basic Research Program of China (2013CBA01900, 2010CB934300, 2011CBA00607, 2011CB932804), National Integrate Circuit Research Program of China (2009ZX02023-003), National Natural Science Foundation of China (61176122, 61106001, 61261160500, 61376006, 61401444), Science and Technology Council of Shanghai (12nm0503701, 13DZ2295700, 12QA1403900, 13ZR1447200, 14ZR1447500). References [1] G. Atwood, Phase change materials for electronic memories, Science 321 (2008) 210–211. [2] M. Wuttig, N. Yamada, Phase-change materials for rewriteable data storage, Nat. Mater. 6 (2007) 824–832. [3] S.R. Ovshinsky, Reversible electrical switching phenomena in disordered structures, Phys. Rev.Lett. 21 (1968) 1450–1453. [4] C. Peng, F. Rao, L.C. Wu, Z.T. Song, Y.·.F. Gu, D. Zhou, H.J. Song, P.X. Yang, J.H. Chu, Homogeneous phase W-Ge-Te material with improved overall phase-change properties for future nonvolatile memory, Acta.Mater. 74 (2014) 49–57. [5] Y.·.H. Joo, J.C. Woo, C.·.I. Kim, Surface reaction effects on dry etching of IGZO thin films in N2/BCl3/Ar plasma, Microelectron. Eng. 112 (2013) 74–79. [6] K.·.H. Kwon, Alexander Efremov, S.J. Yun, Iwoo Chun, K. Kim, Dry etching characteristics of Mo and Al2O3 films in O2/Cl2/Ar inductively coupled plasmas, Thin Solid Films 552 (2014) 105–110. [7] J. Zhou, Y. Chen, W.L. Zhou, X.S. Miao, Z. Yang, N.·.N. Yu, H. Liu, T. Lan, J.B. Yan, Inductively coupled plasma etching for phase-change material with superlattic-like structure in phase change memory device, Appl. Surf. Sci. 280 (2013) 862–867. [8] Z.H. Zhang, S.·.S. Song, Z.T. song, Y. Cheng, C. Peng, L. Zhang, D.C. Cao, X.H. Guo, L.C. Wu, B. Liu, Characteristics and mechanism of Al1.3Sb3Te etched by Cl2/BCl3 inductively coupled plasmas, Microelectron. Eng. 115 (2015) 51–54. [9] Z.H. Zhang, S.·.S. Song, Z.T. song, Y. Cheng, M. Zhu, X.Y. Li, Y.Q. Zhu, X.H. Guo, W.J. Yin, L.C. Wu, B. Liu, S.L. Feng, D. Zhou, Etching of new phase change material Ti0.5Sb2Te3 by Cl2/Ar and CF4/Ar inductively coupled plasmas, Appl. Surf. Sci. 311 (2014) 68–73. [10] Kim Han-Ki, J.W. Bae, T.–.K. Kim, K.–.K. Kim, T.–.Y. Seong, I. Adesida, Inductively coupled plasma reactive ion etching of ZnO using BCl3-based plasmas, J.Vac.Sci.Technol. B 21 (2003) 1273–1277. [11] R.J. Shul, G.B. McClellan, R.D. Briggs, D.J. Rieger, S.J. Pearton, C.R. Abernathy, J.W. Lee, C. Constantine, C. Barratt, High-density plasma etching of compound semiconductors, J. Vac. Sci. Technol. A 15 (1997) 633–637. [12] K.·.Y. Yang, S.·.H. Hong, D.K. Kim, B.k Cheong, H. Lee, Patterning of Ge2Sb2Te5 phase change material using UV nano-imprint lithography, Microelectron.Eng. 84 (2007) 21–24.
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