Advanced Materials Research Vols. 557-559 (2012) pp 1861-1864 © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.557-559.1861
Non-Volatile Memory Behaviour of MFIS Structure with Ba0.7Sr0.3TiO3 Films for Memory Capacitor Application Htet Htet Nwe1, a, Yin Maung Maung2,b , Than Than Win3,c and Ko Ko Kyaw Soe4,d 1
Department of Physics, Dagon University, Yangon, Myanmar
2
Department of Physics, University of Yangon, Yangon, Myanmar
3
Department of Physics, University of Yangon, Yangon, Myanmar 4
University of Yangon, Yangon, Myanmar
a
b
c
[email protected],
[email protected],
[email protected], d
[email protected]
Keywords: Barium Strontium Titanate, MFIS, Dielectric, P-E hysteresis loop
Abstract. The growth of strontium doped barium titanate (BST) powder is performed by solid-state reaction. Microstructural characterization by SEM technique confirms the BST film is successfully formed on p-Si (100) substrate by spin coating technique. Dielectric properties of MFIS (Metal/ Ferroelectric/ Insulator/ Semiconductor) designs are measured by impedance analyzer (LCR meter, QuadTech :1730). P-E (polarization-electric field) hysteresis loops are also measured by applying the same triangular wave electric field in order to allow their application in NVFeRAM (Non-Volatile Ferroelectric Random Access Memory). The maximum remanent polarization density (Pr=38.6µC/cm2) is found at BST film 600˚C. The film exhibits the potential to be a future promising memory. The results obtained from this research are quite interesting, acceptable, credible and applicable in use for NVFeRAM. Introduction Recently, embedded memory in microcontrollers has become necessary to achieve high-speed access time and low-voltage operation for mobile computing. Semiconductor memories such as DRAM and SRAM are commonly utilized; however, these are volatile memory types whose data are lost when the power is shut down. In contrast to volatile memories, the data stored in nonvolatile memory remain when the power is shut down[1]. The ferroelectric random access memory (FeRAM) is one of the alternative nonvolatile memories that can replace the flash memory due to its fast speed, nonvolatility,high endurance, and low operating voltage [2-5]. Barium strontium titanate (BST), a solid solution perovskite, is a potential candidate for integration into microwave devices. BST ferroelectric thin films are attractive for radio frequency and microwave applications due to its high figure of merit, thermal stability and ease of integration into microelectronic circuit [6]. Experimental Growth of Ba(Sr)TiO3 Powder. Barium carbonate (BaCO3), titanium dioxide (TiO2) and strontium carbonate (SrCO3) were mixed by solid state mixed oxide route. This powder was mixed by using agate mortar and calcined at 1200°C for 1h and 30 min in order to decarbonate. Ba(Sr)TiO3 (BST) crystalline powder was sieved with 3-step mesh sieve (100 mesh, 250 mesh, 400 mesh) to get uniform grain/particle size. This powder was operated by ball mill to reduce particle size. Air-jet mill was used to get homogeneous and moisture-less powder. The mixed powder was annealed at 120˚C for 1h and Ba(Sr)TiO3 powder was formed.
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Advanced Materials and Processes II
Preparation of BST Solution. The laboratory-grown BST powder and 2-methoxyethonal (CH3OCH2CH2OH) solvent were used to prepare the BST sol solution. They were mixed and stirred with magnetic stirrer with constant speed (600 rpm) for 5 h to obtain homogeneous BST sol solution. The sol solution was refluxed at 110°C for 4h in a three-neck flask assembly to react the BST powder and boiled CH3OCH2CH2OH solvent. BST sol solution was formed after cool down at room temperature. It was aged for 5h to enhance the viscosity. Thin Film Deposition. SiO2 layer (insulating layer) was thermally formed on p-Si (100) substrate (1cm x 1cm). BST sol solution was deposited on SiO2/Si by Single Wafer Spin Processor. The spin speed or rotational speed was set 4000 rpm and spinning time was 30 sec. To change the sol coating into oxide film, they were annealed at 500°C, 600°C and 700°C in O2-atm for 1h, respectively. Eventually, BST/SiO2/Si cells were formed at different process temperatures. Electro-less Nickel Plating . Ni-conductive layer was formed on front and back (counter) sides of BST film by electro-less Ni-plating. The exposed area (0.4cm x 0.4cm) was set on front side and remaining area was covered with mask. The exposed area (0.6cm x 0.6cm) was also set on back side of the p-Si (100) wafer. After that, Ni-conductive layers were formed by electro-less Ni-plating. After 3 min, it was taken from Ni-solution and dried at room temperature. After removing the mask, Ni-conductive layer was formed. Cu-wire was soldered on front and counter conductive layer. SEM Analysis SEM Images of BST Films. Morphological changes on BST/SiO2/Si structures observed by SEM and surface micrograph of the films were shown in Fig.1(a-c). From the results, the grain sizes were observed to be 0.20 µm, 0.21 µm and 0.32 µm for BST/SiO2/Si films at 500°C, 600°C and 700°C. The surface morphology was examined to be little different. The exact and crowded spherical smooth grain BST films were detected. However, one could be predicted that the BST film on SiO2/Si substrate was of integrity and non-cracking.
(a) (b) (c) Fig 1. SEM microphotographs of BST/SiO2/Si films at (a) 500°C (b) 600˚C (c) 700˚C
Dielectric Properties Dielectric Properties of MFIS Cells with BST Film. The approach of dielectric properties (C-f, and εr-f) were taken to investigate the ferroelectric behaviour of MFIS cell with BST films. C-f characteristics of BST films at different process temperatures were shown in Fig 2(a-c). εr–f variations were also studied and plotted as Fig 3 (a-c). A study of the frequency and temperature dependence of the dielectric properties revealed behaviour characteristics of ionic motion in BST film. It appeared that oxygen vacancy migration promoted as ion jump relaxation at relatively low frequency. In addition, once BST films were successfully polarized at low frequency. As a result of dielectric properties, it was suggested that Sr-dopant may restrict the addition of oxygen vacancy in fabricated films.
Advanced Materials Research Vols. 557-559
1e-9
1.4e-9 Unfitted Fitted
1.2e-9
7e-10
Unfitted Fitted
8e-10
6e-10
Unfitted Fitted
5e-10
8.0e-10 6.0e-10 4.0e-10
Transition Frequency = 5.5 kHz
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Capacitance (F)
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4e-10 Transition Frequency = 8.4 kHz
2e-10
3e-10 Transition Frequency = 8.8 kHz 2e-10 1e-10
BST/SiO2/Si
BST/SiO2/Si
4e-10
BST/SiO2/Si
0 0.0
0
0.0
2.0e+4
4.0e+4
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Applied Frequency (Hz)
Applied Frequency (Hz)
4.0e+4
6.0e+4
8.0e+4
1.0e+5
1.2e+5
Applied Frequency (Hz)
(a) (b) (c) Fig 2. Capacitance - frequency characteristic curves of BST/SiO2/Si films at (a) 500˚C (b) 600˚C (c) 700˚C 4000
1000
BST/SiO2/Si
1400
1500
1000 Transition Frequency = 8.4 kHz 500
BST/SiO2/Si
0
Unfitted Fitted
1600
Dielectric constant
Transition Frequency = 4.9 kHz
Dielectric constant
2000
Unfitted Fitted
2000
3000
Dielectric constant
1800
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1200 1000 800 Transition Frequency = 9.6 kHz
600 400 200
BST/SiO2/Si
0 0
0.0
2.0e+4
4.0e+4
6.0e+4
8.0e+4
Applied Frequency (Hz)
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1.2e+5
0.0
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0.0
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6.0e+4
8.0e+4
1.0e+5
1.2e+5
Applied Frequency (Hz)
(a) (b) (c) Fig 3. Dielectric constant and applied frequency variation curves of BST/SiO2/Si films at (a) 500˚C (b) 600˚C (c) 700˚C Thermal Hysteresis Characteristics (Non-volatile Memory) Hysteresis Loops of BST Films at Different Process Temperatures. Ferroelectricity and non-volatility of fabricated films were interpreted by means of P-E hysteresis loop. Hysteresis loop measurements were performed with Sawyer-Tower circuit without pooling treatment at 100kHz. The generated loop was recorded on oscilloscope (YOKOGAWA ALS10 50MHz) in which the BST cells was served as circuit element. Fig 4(a-c) described the thermal hysteresis loops of MFIS cell with BST film. All hysteresis loops looked non-linear and slim. As the voltage was increased the polarization also increased. As the voltage was decreased with polarization and then back to 0V, only 3/4's of the hysteresis loop was measured. As a result of symmetric hysteresis loop, it was seemed that the loop had a nice rotational symmetry. It means that if the hysteresis loop was rotated by 180˚ it would look almost identical to the memory window, as expected.
(a) (b) (c) Fig 4. P-E hysteresis loops for BST/SiO2/Si films at (a) 500˚C (b) 600˚C (c) 700˚C
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Table 1. 2Pr, Pr, Ps and Ec values for MFIS cells at different process temperatures Sample
Temp(˚C)
2Pr[µC/cm2]
Pr[µC/cm2]
Ps[µC/cm2]
Ec[kV/cm]
BST/SiO2/Si
500 600 700
77.0 77.1 64.2
38.5 38.6 32.1
48.1 48.2 45.0
1.2 1.1 1.0
Summary Fabrication and characterization of barium strontium titanate thin films have been successfully implemented. SEM micrographs of BST films revealed that the alternating sol chemistry was quite feasible for thin film manufacture. In addition, it was observed that grain growth in film was effective in improving Pr. The exact and crowded spherical smooth grain BST films were detected for MFIS structure. It was also predicted that the BST film of MFIS structure was of integrity and non-cracking. A study of frequency and temperature dependence of the dielectric properties revealed behaviour characteristics of ionic motion in BST film. It appeared that oxygen vacancy migration promoted as ion jump relaxation at relatively low frequency. The BST film was polarized at low frequency and it had a great chance to cause the ferroelectricity of low frequency region. As a result of dielectric properties, it was predicted that Sr-modifier may restrict the addition of oxygen in ferroelectric film. As the hysteresis loop picture, it was found that the polarization significantly depended on the electric field. So, the capacitance and charge were also influenced by electric field. Thus all BST films exhibited the ferroelectricity at given process temperature and it can be used as a preliminary prototype cell for non-volatile memory application (eg. 1C of NVFeRAM). Thus, it satisfies the special requirements for the development of memory device of low cost and Eco-friendly. The present research allows more economical coating, technical simplicity and easy adaptability thereby making products that are more compact. In addition, the colloidal solution deposition is found to be simple and suitable method for obtaining uniform, homogeneous and strong adherent films. Acknowledgement The authors would like to thank Professor Dr Khin Mar Kyu, Head of Department of Physics, Dagon University, for her kind permission to carry out this work. References [1] H Hirano, T Honda, N Moriwaki, T Nakakuma, A Inoue, G Nakane, S Chaya, and T Sumi, IEEE Journal of Solid-State Circuits, Vol. 32, No. 5, (1997) p.649 [2] T. Suzuki and E. Tokumits, Jpn. J. Appl. Phys. 41, (2002) p.6886 [3] E. Tokumitsu, K. Okamoto and H. Ishiwara, Jpn. J.Appl. Phys. 40, (2001) p.2917 [4] T. Li, S. T. Hsu, B. D. Ulrich, L. Stecker, D. R. Evans and J. J. Lee, IEEE Electron. Dev. Lett. 23, (2002) p.339 [5] S. M. Yoon and H. Ishiwara, IEEE Trans. Electron. Dev.48, (2001) p.2002 [6] Ghosh D : Tunable microwave devices using BST(barium strontium titanate) and base metal electrodes PhD Dissertation, (2005) (USA : North Carolina State University)
