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Nanofabrication of spintronic devices with ultra small ferromagnetic contacts
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Microelectronic Engineering 85 (2008) 1152–1156 www.elsevier.com/locate/mee
Nanofabrication of spintronic devices with ultra small ferromagnetic contacts Yifang Chen a,*, Yun Zhou b, Ling Wang a, Zheng Cui a, Ejaz Huq a, Genhua Pan b b
a Central Microstructure Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK School of Computing, Communication and Electronics, University of Plymouth, Plymouth, Drake Circus, Plymouth, Devon PL4 8AA, UK
Received 29 September 2007; received in revised form 11 December 2007; accepted 20 December 2007 Available online 31 December 2007
Abstract This paper reports a recently developed nanofabrication process for the fabrication of vertical nanocontacts in ferromagnetic film, combining electron beam lithography and reactive ion etch. Dry etch for nanopinholes in SiNx layer has been studied. As small as 20 nm contacts have been achieved. The developed nanoprocess is not only applicable for such kind of spintronic devices, but also suitable for general nanoelectronic devices. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Electron beam lithography; Reactive ion etch; Nanofabrication; Ferromagnetic contacts
1. Introduction Short and ultra small nanocontacts in between two ferromagnetic films are not only essential in the study of ballistic magnetoresistance (BMR) for spin dependent transport in ballistic regime [1–3], but also promising for the application of future ultra-high-density storage systems (in the terabits/in2 range). Despite the great effort on the scientific investigation of BMR in such kind of system [1– 3], technical development for ferromagnetic nanoconstrictions using reliable nanolithography is still extremely limited. So far, most of the researches about BMR were conducted in the systems fabricated by non-lithography approaches. Our earlier work in the fabrication of nanoconstrictions in ferromagnetic film by nanolithography approach with current in plane (CIP) configuration [4,5] has demonstrated the feasibility of nanofabrication for ultra small nanocontacts. However, for the possible application of ferromagnetic nanoconstrictions in a read head for example, vertical configuration with current perpendicular to plane (CPP) is demanded. This paper reports our *
Corresponding author. Tel.: +44 1235 445159; fax: +44 1235446283. E-mail address: [email protected] (Y. Chen).
0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.12.027
recent progress in the development of nanofabrication for ferromagnetic nanoconstriction with the CPP configuration, as schematically shown in Fig. 1e. The nanofabrication in this effort involves the state-of-the-art electron beam lithography, reactive ion etch (RIE) for nanostructures and high quality thin film deposition, which will be discussed in details in this paper. 2. Experiments and results In the fabrication of CPP devices, the key part is to fabricate the ultra small contacts of two ferromagnetic films so that the electronic transport through the constriction is maintained in the ballistic regime. Fig. 1 schematically describes the process flow. On the h1 0 0i surfaces of normal silicon wafers, a film stack (Ta 5 nm/Cu 100 nm/Ta 5 nm/ NiFe 20 nm) was first deposited by a magnetron sputter coater. This step establishes the bottom film and electrode. A thin SiNx film ranging from 50 nm to 200 nm was then deposited by plasma enhanced chemical vapor deposition (PECVD). Electron beam lithography was first applied for defining the nano pinhole in the PMMA resist coated on a SiNx film underneath which there is a pre-deposited
Y. Chen et al. / Microelectronic Engineering 85 (2008) 1152–1156
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Fig. 1. A schematic diagram of the process flow to make a electron spintronic device with vertical contact of ferromagnetic films as shown in (e) as the central part.
ferromagnetic film. RIE was then undertaken to transfer the pinhole into the SiNx film as an insulating layer. After stripping off the resist, another ferromagnetic film is deposited as the top part of the device. During the EBL process, high precision registration is required to locate the pinhole onto the required area in which the bottom ferromagnetic film stays. 2.1. Electron beam lithography for defining the pinholes The state-of-the-art electron beam lithography (EBL) with high precision registration was first carried out to replicate nanosize pinholes in a 150 nm thick PMMA resist by a high resolution vector beam writer (VB6 HR) supplied by Vistech Microsystem Ltd of UK. Fig. 2a presents the size of the holes under various exposure doses after EBL. Since the pinhole size in PMMA is not necessarily the same as in SiNx layer, three different digital sizes were designed, 100 nm, 50 nm and 30 nm. The three curves in Fig. 2a correspond to these three designed dimensions. The sample with patterned pinholes are immediately subjected to a reactive ion etch for opening holes in SiNx layer.
2.2. Reactive ion etch for pinholes in SiNx layer The key part of this technique is to make as small as possible pinholes in SiNx layer. Using the patterned PMMA as etch mask, the RIE process was carried out to image the pinholes from the PMMA layer into the SiNx film by a SYS90 dry etcher supplied by oxford plasma technology (OPT). Fluorine based gas was applied. Careful study of RIE property has been undertaken to achieve ultra small pinholes in SiNx film. First of all, the etch selectivity of SiNx over PMMA was measured under two rf powers, 150 W and 250 W. The results are concluded in Table 1. It can be seen that higher power gives rise to higher selectivity; hence 250 W was used for the RIE on SiNx throughout the work. The effect of the etch time on the profile of the etched structure in SiNx was investigated. As shown in Fig. 3, the sidewall profile of SiNx layer can be very different for different etch time. For normal etch time (2.5 min in this particular case) in CHF3, a square like trench is formed as shown in Fig. 3a. However, if the etch time is reduced slightly, for example by 20%, a V-groove profile can be obtained as
250
250
200
100 nm
150 100
50 nm
50
Pinhole diameter (nm)
Pinhole diameter (nm)
a
100 nm
b 200 150
50 nm
100 50
20 nm
20 nm 0
0 0
2000
4000
6000
8000
Dose (uC/cm2)
10000 12000
0
2000
4000
6000
8000
10000 12000
Dose (uC/cm2)
Fig. 2. The pinhole size changing with various exposure dose of e-beam. (a) The size in PMMA layer after EBL. (b) The size in the SiNx layer after EBL and RIE. The three curves in each plot correspond to three digitally designed pinhole size. They are 100 nm, 50 nm and 20 nm, respectively.
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Table 1 Comparison of etch rates between PMMA and SiNx under two different rf power rf Power (W)
PMMA etch rate (nm/min)
SiNx etch rate (nm/min)
Selectivity (SiNx/ PMMA)
150 250
1.87 3.4
4.87 16.6
2.6 4.9
shown in Fig. 3b. Therefore, well control of etch time helps to achieve smaller pinhole size, using the advantage in Vgroove shape in the SiNx layer after the RIE process. It was noticed that the PMMA layer also shows triangular like profile in the trench part, which is rather common and can be easily explained by the RIE of CHF3 plasma on the corners. Fig. 4a–c shows the cross sectional view of the formed pinholes in SiNx, corresponding to three different designed sizes, (a) 100 nm, (b) 50 nm and (c) 30 nm. The rough surface on the top is due to the remaining PMMA resist after plasma etch. A complete range of pinhole diameters are presented in Fig. 2b. As small as 20 nm nano pinholes have been achieved. After the RIE process, the remaining PMMA is easily stripped off in warm acetone, leaving a very clear surface for the subsequent film deposition. 2.3. Formation of nanocontacts in ferromagnetic film by thin film deposition Ferromagnetic film was deposited by a Nordico ac magnetron sputter coater. To ensure a good adhesion, plasma clean in oxygen is first carried out to further remove residual resists. A top layer with NiFe 20 nm/Ta 5 nm/Cu
100 nm/Ta 5 nm is sputter coated. In the early stage of this work, testing samples were used to prove the formation of nanocontacts through the SiNx pinholes, using Au film instead. Fig. 4d–f shows the deposited metallic dots of Au film on Si after wet etching the SiNx. As small as 20– 30 nm contacts are observed by a high resolution scanning electron microscope, Hitachi S-4000 SEM. The formations of nanocontacts are further proved by the inspection of the cross section by the SEM, as shown in Fig. 5. Despite the deep and ultra small pinholes, the metallic film can still be grown into them. The pinhole arrays presented above are used for processing development only. To make real device, only one pinhole is needed. The layout of the device is schematically described by the diagram in Fig. 6. First, a multilayer of Ta 5/Cu 100/Ta 5/NiFe 10 (in nm) was deposited onto a thermally oxidized Si substrate. A bottom Ta/Cu/Ta electrode was then patterned by a combination of photolithography and Ar ion milling. Subsequently, a Ta/Cu/Ta/NiFe element was patterned, and after patterning the nanopillar, MgO was sputtered for isolating the pillar from the rest of the device electrodes, and left only the pillar exposed, which was then coated by a thin SiNx film. After the pinhole made in the SiNx layer, another ferromagnetic film, e.g. CoFe, was deposited as the top part of the device. Finally, a Ta/Cu/Ta multilayer was sputtered as a top electrode using a lift-off process. Four point probe measurements were carried out in the CPP geometry at room temperature with an external magnetic field applied along the easy axis. Using the optimized processing condition, we have successfully fabricated metallic nanocontacts, which are currently being characterized. Due to the space limitation
Fig. 3. The effect of etch time on the profiles in SiNx trench created by a RIE process using CHF3 gas. It was found that the etch time can substantially influence the slope of the sidewall of SiNx. For opening a vertical trench as shown in (a), an etch time of 2.5 min was used. However, if slightly under-etch time (reduced by 20%) was used, its profile becomes V-groove, which is beneficial for reducing the pin-hole size. The RIE process also files the corner of PMMA layer, leaving triangular shape as shown in the PMMA layer.
Y. Chen et al. / Microelectronic Engineering 85 (2008) 1152–1156
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Fig. 4. Cross sectional views of the pinholes in SiNx layer with three different designed hole size in EBL: (a) 100 nm, (b) 50 nm and (c) 30 nm. The RIE process was carried out on the SiNx for all these three sizes in the same sample. 250 W of rf power with CHF3 gas (30 sccm) was used. The time length of the RIE etch is about 2 min. V-groove shape is observed for all these sizes. (d–f) show the areas of pinholes at the bottom after removing the SiNx layer by wet etch. The dots are made by the materials of 5 nm Cr/5 nm Au. Its area reflects the size of the pinholes.
Fig. 5. Demonstration of metallic nanoconstrictions in SiNx layer. (a) 100 nm holes are designed and (b) the sub 50 nm is designed. The metallic films in these particular photos are Au deposited by thermal evaporation.
and the scope of this paper, the measurement results will be published elsewhere. 3. Conclusion
Fig. 6. A schematic diagram of the device layout (the central part) with an aligned vertical nanoconstriction in the centre. The top and bottom electrode were electrically separated by a layer of silica grown by PE CVD.
We have developed a reliable nanoprocess for the fabrication of nanosize spintronic devices in ferromagnetic films with ultra small contacts. Such a new device configuration with the current perpendicular to plane exhibits a wealth of magnetotransport properties to be discovered. It is expected that the magneto resistance (MR) existing in such a CPP nanocontact shall posses potential applications in high density data storage devices. The developed process is not only applicable for the fabrication of the electron
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spintronic devices, but also for other nanoelectronic devices such as single electron tunneling transisters. References [1] N. Garcia, M. Munoz, Y.W. Zhao, Phys. Rev. Lett. 82 (14) (1999) 2923. [2] Susan Z. Hua, Harsh Deep Chopra, Phys. Rev. B67 (2003) 060401(R).
[3] C. Ruster, T. Borzenko, C. Gould, G. Schmidt, L.W. Molenkamp, Phys. Rev. Lett. 91 (21) (2003) 216602-1. [4] Yifang Chen, Yun Zhou, Zhengqi Lu, Xudi Wang, Zheng Cui, et al, in: Proceedings of the 32nd International Conference on Micro- and Nano-Engineering 2005, 17–20 September, Barcelona, Spain, 2006. [5] Z.Q. Lu, Y. Zhou, Y.Q. Du, R. Moate, D. Wilton, G.H. Pan, Y. Chen, Z. Cui, Appl. Phys. Lett. 88 (14) (2006). Art. No. 142507 APR 3.
Microelectronic Engineering 85 (2008) 1152–1156 www.elsevier.com/locate/mee
Nanofabrication of spintronic devices with ultra small ferromagnetic contacts Yifang Chen a,*, Yun Zhou b, Ling Wang a, Zheng Cui a, Ejaz Huq a, Genhua Pan b b
a Central Microstructure Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK School of Computing, Communication and Electronics, University of Plymouth, Plymouth, Drake Circus, Plymouth, Devon PL4 8AA, UK
Received 29 September 2007; received in revised form 11 December 2007; accepted 20 December 2007 Available online 31 December 2007
Abstract This paper reports a recently developed nanofabrication process for the fabrication of vertical nanocontacts in ferromagnetic film, combining electron beam lithography and reactive ion etch. Dry etch for nanopinholes in SiNx layer has been studied. As small as 20 nm contacts have been achieved. The developed nanoprocess is not only applicable for such kind of spintronic devices, but also suitable for general nanoelectronic devices. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Electron beam lithography; Reactive ion etch; Nanofabrication; Ferromagnetic contacts
1. Introduction Short and ultra small nanocontacts in between two ferromagnetic films are not only essential in the study of ballistic magnetoresistance (BMR) for spin dependent transport in ballistic regime [1–3], but also promising for the application of future ultra-high-density storage systems (in the terabits/in2 range). Despite the great effort on the scientific investigation of BMR in such kind of system [1– 3], technical development for ferromagnetic nanoconstrictions using reliable nanolithography is still extremely limited. So far, most of the researches about BMR were conducted in the systems fabricated by non-lithography approaches. Our earlier work in the fabrication of nanoconstrictions in ferromagnetic film by nanolithography approach with current in plane (CIP) configuration [4,5] has demonstrated the feasibility of nanofabrication for ultra small nanocontacts. However, for the possible application of ferromagnetic nanoconstrictions in a read head for example, vertical configuration with current perpendicular to plane (CPP) is demanded. This paper reports our *
Corresponding author. Tel.: +44 1235 445159; fax: +44 1235446283. E-mail address: [email protected] (Y. Chen).
0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.12.027
recent progress in the development of nanofabrication for ferromagnetic nanoconstriction with the CPP configuration, as schematically shown in Fig. 1e. The nanofabrication in this effort involves the state-of-the-art electron beam lithography, reactive ion etch (RIE) for nanostructures and high quality thin film deposition, which will be discussed in details in this paper. 2. Experiments and results In the fabrication of CPP devices, the key part is to fabricate the ultra small contacts of two ferromagnetic films so that the electronic transport through the constriction is maintained in the ballistic regime. Fig. 1 schematically describes the process flow. On the h1 0 0i surfaces of normal silicon wafers, a film stack (Ta 5 nm/Cu 100 nm/Ta 5 nm/ NiFe 20 nm) was first deposited by a magnetron sputter coater. This step establishes the bottom film and electrode. A thin SiNx film ranging from 50 nm to 200 nm was then deposited by plasma enhanced chemical vapor deposition (PECVD). Electron beam lithography was first applied for defining the nano pinhole in the PMMA resist coated on a SiNx film underneath which there is a pre-deposited
Y. Chen et al. / Microelectronic Engineering 85 (2008) 1152–1156
1153
Fig. 1. A schematic diagram of the process flow to make a electron spintronic device with vertical contact of ferromagnetic films as shown in (e) as the central part.
ferromagnetic film. RIE was then undertaken to transfer the pinhole into the SiNx film as an insulating layer. After stripping off the resist, another ferromagnetic film is deposited as the top part of the device. During the EBL process, high precision registration is required to locate the pinhole onto the required area in which the bottom ferromagnetic film stays. 2.1. Electron beam lithography for defining the pinholes The state-of-the-art electron beam lithography (EBL) with high precision registration was first carried out to replicate nanosize pinholes in a 150 nm thick PMMA resist by a high resolution vector beam writer (VB6 HR) supplied by Vistech Microsystem Ltd of UK. Fig. 2a presents the size of the holes under various exposure doses after EBL. Since the pinhole size in PMMA is not necessarily the same as in SiNx layer, three different digital sizes were designed, 100 nm, 50 nm and 30 nm. The three curves in Fig. 2a correspond to these three designed dimensions. The sample with patterned pinholes are immediately subjected to a reactive ion etch for opening holes in SiNx layer.
2.2. Reactive ion etch for pinholes in SiNx layer The key part of this technique is to make as small as possible pinholes in SiNx layer. Using the patterned PMMA as etch mask, the RIE process was carried out to image the pinholes from the PMMA layer into the SiNx film by a SYS90 dry etcher supplied by oxford plasma technology (OPT). Fluorine based gas was applied. Careful study of RIE property has been undertaken to achieve ultra small pinholes in SiNx film. First of all, the etch selectivity of SiNx over PMMA was measured under two rf powers, 150 W and 250 W. The results are concluded in Table 1. It can be seen that higher power gives rise to higher selectivity; hence 250 W was used for the RIE on SiNx throughout the work. The effect of the etch time on the profile of the etched structure in SiNx was investigated. As shown in Fig. 3, the sidewall profile of SiNx layer can be very different for different etch time. For normal etch time (2.5 min in this particular case) in CHF3, a square like trench is formed as shown in Fig. 3a. However, if the etch time is reduced slightly, for example by 20%, a V-groove profile can be obtained as
250
250
200
100 nm
150 100
50 nm
50
Pinhole diameter (nm)
Pinhole diameter (nm)
a
100 nm
b 200 150
50 nm
100 50
20 nm
20 nm 0
0 0
2000
4000
6000
8000
Dose (uC/cm2)
10000 12000
0
2000
4000
6000
8000
10000 12000
Dose (uC/cm2)
Fig. 2. The pinhole size changing with various exposure dose of e-beam. (a) The size in PMMA layer after EBL. (b) The size in the SiNx layer after EBL and RIE. The three curves in each plot correspond to three digitally designed pinhole size. They are 100 nm, 50 nm and 20 nm, respectively.
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Table 1 Comparison of etch rates between PMMA and SiNx under two different rf power rf Power (W)
PMMA etch rate (nm/min)
SiNx etch rate (nm/min)
Selectivity (SiNx/ PMMA)
150 250
1.87 3.4
4.87 16.6
2.6 4.9
shown in Fig. 3b. Therefore, well control of etch time helps to achieve smaller pinhole size, using the advantage in Vgroove shape in the SiNx layer after the RIE process. It was noticed that the PMMA layer also shows triangular like profile in the trench part, which is rather common and can be easily explained by the RIE of CHF3 plasma on the corners. Fig. 4a–c shows the cross sectional view of the formed pinholes in SiNx, corresponding to three different designed sizes, (a) 100 nm, (b) 50 nm and (c) 30 nm. The rough surface on the top is due to the remaining PMMA resist after plasma etch. A complete range of pinhole diameters are presented in Fig. 2b. As small as 20 nm nano pinholes have been achieved. After the RIE process, the remaining PMMA is easily stripped off in warm acetone, leaving a very clear surface for the subsequent film deposition. 2.3. Formation of nanocontacts in ferromagnetic film by thin film deposition Ferromagnetic film was deposited by a Nordico ac magnetron sputter coater. To ensure a good adhesion, plasma clean in oxygen is first carried out to further remove residual resists. A top layer with NiFe 20 nm/Ta 5 nm/Cu
100 nm/Ta 5 nm is sputter coated. In the early stage of this work, testing samples were used to prove the formation of nanocontacts through the SiNx pinholes, using Au film instead. Fig. 4d–f shows the deposited metallic dots of Au film on Si after wet etching the SiNx. As small as 20– 30 nm contacts are observed by a high resolution scanning electron microscope, Hitachi S-4000 SEM. The formations of nanocontacts are further proved by the inspection of the cross section by the SEM, as shown in Fig. 5. Despite the deep and ultra small pinholes, the metallic film can still be grown into them. The pinhole arrays presented above are used for processing development only. To make real device, only one pinhole is needed. The layout of the device is schematically described by the diagram in Fig. 6. First, a multilayer of Ta 5/Cu 100/Ta 5/NiFe 10 (in nm) was deposited onto a thermally oxidized Si substrate. A bottom Ta/Cu/Ta electrode was then patterned by a combination of photolithography and Ar ion milling. Subsequently, a Ta/Cu/Ta/NiFe element was patterned, and after patterning the nanopillar, MgO was sputtered for isolating the pillar from the rest of the device electrodes, and left only the pillar exposed, which was then coated by a thin SiNx film. After the pinhole made in the SiNx layer, another ferromagnetic film, e.g. CoFe, was deposited as the top part of the device. Finally, a Ta/Cu/Ta multilayer was sputtered as a top electrode using a lift-off process. Four point probe measurements were carried out in the CPP geometry at room temperature with an external magnetic field applied along the easy axis. Using the optimized processing condition, we have successfully fabricated metallic nanocontacts, which are currently being characterized. Due to the space limitation
Fig. 3. The effect of etch time on the profiles in SiNx trench created by a RIE process using CHF3 gas. It was found that the etch time can substantially influence the slope of the sidewall of SiNx. For opening a vertical trench as shown in (a), an etch time of 2.5 min was used. However, if slightly under-etch time (reduced by 20%) was used, its profile becomes V-groove, which is beneficial for reducing the pin-hole size. The RIE process also files the corner of PMMA layer, leaving triangular shape as shown in the PMMA layer.
Y. Chen et al. / Microelectronic Engineering 85 (2008) 1152–1156
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Fig. 4. Cross sectional views of the pinholes in SiNx layer with three different designed hole size in EBL: (a) 100 nm, (b) 50 nm and (c) 30 nm. The RIE process was carried out on the SiNx for all these three sizes in the same sample. 250 W of rf power with CHF3 gas (30 sccm) was used. The time length of the RIE etch is about 2 min. V-groove shape is observed for all these sizes. (d–f) show the areas of pinholes at the bottom after removing the SiNx layer by wet etch. The dots are made by the materials of 5 nm Cr/5 nm Au. Its area reflects the size of the pinholes.
Fig. 5. Demonstration of metallic nanoconstrictions in SiNx layer. (a) 100 nm holes are designed and (b) the sub 50 nm is designed. The metallic films in these particular photos are Au deposited by thermal evaporation.
and the scope of this paper, the measurement results will be published elsewhere. 3. Conclusion
Fig. 6. A schematic diagram of the device layout (the central part) with an aligned vertical nanoconstriction in the centre. The top and bottom electrode were electrically separated by a layer of silica grown by PE CVD.
We have developed a reliable nanoprocess for the fabrication of nanosize spintronic devices in ferromagnetic films with ultra small contacts. Such a new device configuration with the current perpendicular to plane exhibits a wealth of magnetotransport properties to be discovered. It is expected that the magneto resistance (MR) existing in such a CPP nanocontact shall posses potential applications in high density data storage devices. The developed process is not only applicable for the fabrication of the electron
1156
Y. Chen et al. / Microelectronic Engineering 85 (2008) 1152–1156
spintronic devices, but also for other nanoelectronic devices such as single electron tunneling transisters. References [1] N. Garcia, M. Munoz, Y.W. Zhao, Phys. Rev. Lett. 82 (14) (1999) 2923. [2] Susan Z. Hua, Harsh Deep Chopra, Phys. Rev. B67 (2003) 060401(R).
[3] C. Ruster, T. Borzenko, C. Gould, G. Schmidt, L.W. Molenkamp, Phys. Rev. Lett. 91 (21) (2003) 216602-1. [4] Yifang Chen, Yun Zhou, Zhengqi Lu, Xudi Wang, Zheng Cui, et al, in: Proceedings of the 32nd International Conference on Micro- and Nano-Engineering 2005, 17–20 September, Barcelona, Spain, 2006. [5] Z.Q. Lu, Y. Zhou, Y.Q. Du, R. Moate, D. Wilton, G.H. Pan, Y. Chen, Z. Cui, Appl. Phys. Lett. 88 (14) (2006). Art. No. 142507 APR 3.