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STM surface modification of the Si–SiO2–polymer system
Microelectronic Engineering 69 (2003) 399–404 www.elsevier.com / locate / mee
STM surface modification of the Si–SiO 2 –polymer system V.M. Kornilov*, A.N. Lachinov Institute of Molecular and Crystal Physics, Ufa Research Center, Russian Academy of Sciences, 450075 Ufa, Russia
Abstract This paper presents the results of a STM investigation of the Si–SiO 2 and Si–SiO 2 –polymer systems in air. Depending on the scanning parameters, such as the applied voltage and tunneling current, a modification of the Si–SiO 2 surface was observed during the experiments. The possibility of a reversible modification was demonstrated. A thin polymer film was used to exclude the adsorption–desorption and electrochemical processes on the Si surface. Modification of the Si–SiO 2 – polymer surface was observed at scanning parameters similar to those used for modification of the Si–SiO 2 system. The electronic mechanism of the surface modification based on tunneling of the charge through the oxide layer and its influence on the STM tunneling current is discussed. 2003 Elsevier B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Si–SiO 2 interfaces; Polymer; Charge transfer
1. Introduction Si–SiO 2 interfaces possess unique electronic properties. The peculiarities of charge accumulation and transfer near this boundary underlie the operation of a wide range of semiconductor devices. Native oxide on the Si surface makes it difficult to measure and maintain the tunneling current. Therefore, STM investigations of such a surface require the application of various methods for etching and passivation of the silicon surface. The possibility of the STM study of the Si–SiO 2 interface properties was then examined. Results of studies have shown the possibility of silicon surface modification during STM studies in
*Corresponding author. E-mail address: [email protected] (V.M. Kornilov).
air [1–4]. An explanation for this is oxidation on the Si surface near the flowing current. However, an analysis of these reports shows that the oxidation model cannot explain many of the surface modification phenomena.
2. Reversible surface modification The aim of this work was to study the mechanism of silicon surface modification using the STM method in air. Monocrystal wafers of n-type Si with a native oxide layer were used for the investigations. The work was carried out using an SMM-2000T scanning multimicroscope (ZAO KPD, Moscow, Zelenograd) operating in the scanning tunneling microscope mode. The probe was prepared by the oblique cutting of a platinum wire. Fig. 1 presents the STM image of the initial Si surface. The silicon is n-type with a (111) orienta-
0167-9317 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00327-7
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V.M. Kornilov, A.N. Lachinov / Microelectronic Engineering 69 (2003) 399–404
Fig. 1. Typical image of the initial (unmodified) surface of the silicon with a native SiO 2 layer.
tion. The Si surface, which was polished during the manufacturing process, was not subjected to additional treatment.
The thickness of the oxide film was less than 2 nm, estimated by the ellipsometry method. Such an oxide film thickness allows one to perform measurements in the constant tunneling current mode. The current flowing through the system may be represented as a superposition of a few components: the current of direct tunneling, Fouler–Nordheim tunneling, and the surface current. Changes in the scanning parameters emphasize certain current components, which influences the image. Similar results have been obtained previously [5]. Fig. 2 presents a typical image of the modified areas of the Si surface. It was established that a stable image might be obtained under certain scanning parameters: 3 , Uv is , 6 V, where Uvis is the bias voltage applied to the sample during normal STM imaging; and 10 , I , 100 pA, where I is the tunneling current. Surface relief modification takes place if the modification voltage Umod . Uv is 1 0.4 V. A cavity forms in the STM image during scanning at a positive Umod voltage applied to the sample. Visualization of the peculiarity obtained is performed by means of scanning a larger area at the Uvis of the
Fig. 2. Image of the silicon surface modified under a positive voltage (morphology, three-dimensional presentation and profile). Modification voltage, 4.5 V; visualization voltage, 3.26 V. Modified area size 0.830.8 mm, 6–8 nm deep.
V.M. Kornilov, A.N. Lachinov / Microelectronic Engineering 69 (2003) 399–404
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Fig. 3. Image of the silicon surface modified under a negative voltage (morphology, three-dimensional presentation and profile). Modification voltage, 24.5 V; visualization voltage, 23.34 V. Modified area sizes 0.230.2, 0.530.5 and 131 mm; height 5–8 nm.
same polarity. If the same procedure is carried out in the reverse polarity (minus on the sample) the modified area is displayed as a protrusion. Fig. 3 presents an image of the Si surface with three separate modified areas. These areas were modified one after another with intermediate control of each mark as described above. It is interesting to note that the scanning parameters were almost the same for measurements made with different voltage polarities. However, an essential distinction was in the character of the modification obtained. Modified areas obtained in negative polarity look like structured protrusions, i.e. are composed of smaller elements (Fig. 3, profile). Those obtained in the positive polarity look like cavities and no structure is observed (Fig. 2, profile). To interpret the data obtained let us consider the energy structure of the multi-layer system: STMprobe–adsorbate layer–SiO 2 –Si (Fig. 4). The same structures are used as elements when fabricating nonvolatile memory, the operation principle of which is based on the capture of the injected charge in the insulator layer [6]. The charge captured in the insulator (adsorbate) layer can sufficiently affect the
tunneling parameters, which is reflected in the STM image. When a negative voltage is applied to an n-type semiconductor substrate, an enriched layer appears near the Si–SiO 2 interface. At high voltage, the charge carriers tunnel through the oxide layer following the Fouler–Nordheim law. This occurs when the voltage applied exceeds the height of the potential barrier on the Si–SiO 2 interface (3.2 V). The charge captured in the adsorbate layer can reduce the electron affinity and increase additional emission components of the current. The latter may be registered by the STM as a protrusion on the surface. When a positive voltage is applied to an n-type semiconductor substrate, a depletion or even an inverse layer appears near the Si–SiO 2 interface. This may reduce the tunneling probability, which can be registered by the STM as a cavity. The suggested model of the modification of the interface charge state predicts the possibilities of writing, reading, erasing and rewriting information by means of a polarity change. All of these possibilities were realized in this work (Fig. 5). Rewriting of the information was performed as
402
V.M. Kornilov, A.N. Lachinov / Microelectronic Engineering 69 (2003) 399–404
Fig. 4. Energy diagrams of the experimental structure under different voltage U polarities [6]. Upper scheme corresponds to a negative voltage applied to the sample, and the lower to a positive voltage applied to the sample. Arrows indicate charge carrier movement. EF , Fermi level.
follows. First, the image of the modified surface under a negative voltage was obtained (square mark in Fig. 5a). This area was then scanned under a positive voltage applied to the sample (Fig. 5b). After the scanning, the mark faded away. Subsequent scanning under a negative voltage revealed no mark (Fig. 5c). Fig. 5d presents a new mark written at the location of the previous one. The electrochemical processes of depassivation or oxidation alone can hardly explain Si surface modification during STM investigations. A structure of the insulator–oxide–semiconductor type forms on
the investigated surface with the adsorbate layer as the insulator. Injected through the oxide layer, charge carriers are captured in the insulator layer. The STM methods allow one to stimulate the injection process and to register the resulting redistribution of the charge near the Si–SiO 2 surface.
3. The Si–SiO 2 –polymer system: surface modification Usually, the Si surface modification obtained
Fig. 5. Illustration of the possibility of information rewriting on the same area of the sample. The dashed line outlines the modified area of the Si surface. (a) Modified area; modification voltage, 24.5 V; visualization voltage, 23.14 V. (b) The same area; visualization voltage, 13.3 V. (c) The same area; visualization voltage, 23.14 V. (d) New mark created at the location of the previous one under the same conditions.
V.M. Kornilov, A.N. Lachinov / Microelectronic Engineering 69 (2003) 399–404
during STM measurements is explained by irreversible oxidation [1–4,7,8]. The adsorbate molecular layer is considered to assist with the oxidation. However, different functions are attributed to this layer in different reports. Thus the role of this layer in the formation of structures on the surface of the Si–SiO 2 system is not clear. In this work a thin polymer film was used to exclude the adsorption– desorption processes on the Si surface. It was shown previously that a poly(heteroarylene) film up to 100 nm thick deposited on a conductive surface can be investigated by the STM method [9]. It is well known that the processes at the metal–polymer interface significantly affect the electronic properties of thin polymer films, particularly by influencing the characteristics of the potential barrier at the interface. This makes it possible to use such films as unique charge sensors [10,11]. The polymer film was obtained from a solution in cyclohexanone by centrifugation on a silicon surface. The thickness of the polymer films used in the experiments varied from 40 to 60 nm. Fig. 6 presents an image of a polymer surface after the preliminary scanning of four areas. The operation
403
order was as follows. Four small areas of the surface were initially scanned with a high applied voltage (modification mode, minus on the sample). Then a larger area containing the four smaller modified areas was scanned at a lower applied voltage (visualization mode). In the visualization mode a stable image can be obtained at 3 , Uvis , 6 V, 10 , I , 100 pA. These parameters are similar to those used for the investigation of the initial Si–SiO 2 surface [12]. If the same procedures were carried out with the polarity reversed (plus on the sample), the modified areas look like depressions (Fig. 7). The conditions for ‘‘rewriting’’ on the same location were determined. When the negative polarity is on an n-type semiconductor, the enriched layer forms at the semiconductor–polymer interface. Scanning under high voltage induces tunneling of the charge carriers through the oxide layer following the Fouler–Nordheim law. The carriers are partially captured on the polymer film traps. Another part of the carriers forms the tunneling current. The captured charge may lead to the formation of deep electron states and even a narrow subband near the middle of the polymer band gap.
Fig. 6. Image of four modified (under a negative voltage) areas of the surface of a polymer film deposited on silicon. Morphology, three-dimensional presentation and profile. Modifying voltage, 24.5 V; visualization voltage, 23.4 V. Area 0.530.5 mm, high relief 100–130 nm.
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V.M. Kornilov, A.N. Lachinov / Microelectronic Engineering 69 (2003) 399–404
Fig. 7. Image of the modified (under a positive voltage) area of the surface of a polymer film deposited on silicon. Morphology and profile. Modifying voltage, 4.5 V; visualization voltage, 3.4 V.
When a positive voltage is applied to the sample, a depletion layer forms near the polymer interface, which may lead to a decrease in the tunneling probability between the probe and the sample. Thus it has been shown that the Si surface modification occurring during STM measurements is significantly affected by charge processes at the interfaces. The electron mechanism of the surface modification makes it possible to use STM for nanoscale writing, rewriting and reading information in Si–SiO 2 –polymer structures.
References [1] J.A. Dagata, J. Schneir, H.H. Harary et al., Appl. Phys. Lett. 56 (1990) 2001. [2] P. Fay, R.T. Brockenbrough, G.J. Abeln et al., J. Appl. Phys. 75 (1994) 7545.
[3] L.N. Bolotov, V.A. Kozlov, I.V. Makarenko, A.N. Titkov, Phys. Techn. Semicond. 27 (1993) 1375. [4] N. Barniol, F. Perez-Murano, G. Abadal, J.H. Ye, X. Aymerich, Microelectron. Eng. 27 (1995) 27. [5] S.Yu. Vassiliev, A.V. Denisov, Techn. Phys. 70 (2000) 100. [6] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. [7] J.W. Lyding, T.-C. Shen, J.S. Hubacek et al., Appl. Phys. Lett. 64 (1994) 2010. [8] G. Abadal, F. Perez-Murano, N. Barniol, X. Aymerich, Appl. Phys. A 66 (1998) 791. [9] V.C. Kornilov, A.N. Lachinov, Techn. Phys. Lett. 26 (2000) 37. [10] V.C. Kornilov, A.N. Lachinov, J. Exp. Theor. Phys. 84 (1997) 833. [11] A.N. Lachinov, T.G. Zagurenko, V.C. Kornilov et al., Phys. Solid State 42 (2000) 1935. [12] A.N. Lachinov, V.C. Kornilov, in: International Workshop ‘‘Scanning Probe Microscopy—2002’’. Book of Abstracts, Nizhni Novgorod, 2002.
STM surface modification of the Si–SiO 2 –polymer system V.M. Kornilov*, A.N. Lachinov Institute of Molecular and Crystal Physics, Ufa Research Center, Russian Academy of Sciences, 450075 Ufa, Russia
Abstract This paper presents the results of a STM investigation of the Si–SiO 2 and Si–SiO 2 –polymer systems in air. Depending on the scanning parameters, such as the applied voltage and tunneling current, a modification of the Si–SiO 2 surface was observed during the experiments. The possibility of a reversible modification was demonstrated. A thin polymer film was used to exclude the adsorption–desorption and electrochemical processes on the Si surface. Modification of the Si–SiO 2 – polymer surface was observed at scanning parameters similar to those used for modification of the Si–SiO 2 system. The electronic mechanism of the surface modification based on tunneling of the charge through the oxide layer and its influence on the STM tunneling current is discussed. 2003 Elsevier B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Si–SiO 2 interfaces; Polymer; Charge transfer
1. Introduction Si–SiO 2 interfaces possess unique electronic properties. The peculiarities of charge accumulation and transfer near this boundary underlie the operation of a wide range of semiconductor devices. Native oxide on the Si surface makes it difficult to measure and maintain the tunneling current. Therefore, STM investigations of such a surface require the application of various methods for etching and passivation of the silicon surface. The possibility of the STM study of the Si–SiO 2 interface properties was then examined. Results of studies have shown the possibility of silicon surface modification during STM studies in
*Corresponding author. E-mail address: [email protected] (V.M. Kornilov).
air [1–4]. An explanation for this is oxidation on the Si surface near the flowing current. However, an analysis of these reports shows that the oxidation model cannot explain many of the surface modification phenomena.
2. Reversible surface modification The aim of this work was to study the mechanism of silicon surface modification using the STM method in air. Monocrystal wafers of n-type Si with a native oxide layer were used for the investigations. The work was carried out using an SMM-2000T scanning multimicroscope (ZAO KPD, Moscow, Zelenograd) operating in the scanning tunneling microscope mode. The probe was prepared by the oblique cutting of a platinum wire. Fig. 1 presents the STM image of the initial Si surface. The silicon is n-type with a (111) orienta-
0167-9317 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00327-7
400
V.M. Kornilov, A.N. Lachinov / Microelectronic Engineering 69 (2003) 399–404
Fig. 1. Typical image of the initial (unmodified) surface of the silicon with a native SiO 2 layer.
tion. The Si surface, which was polished during the manufacturing process, was not subjected to additional treatment.
The thickness of the oxide film was less than 2 nm, estimated by the ellipsometry method. Such an oxide film thickness allows one to perform measurements in the constant tunneling current mode. The current flowing through the system may be represented as a superposition of a few components: the current of direct tunneling, Fouler–Nordheim tunneling, and the surface current. Changes in the scanning parameters emphasize certain current components, which influences the image. Similar results have been obtained previously [5]. Fig. 2 presents a typical image of the modified areas of the Si surface. It was established that a stable image might be obtained under certain scanning parameters: 3 , Uv is , 6 V, where Uvis is the bias voltage applied to the sample during normal STM imaging; and 10 , I , 100 pA, where I is the tunneling current. Surface relief modification takes place if the modification voltage Umod . Uv is 1 0.4 V. A cavity forms in the STM image during scanning at a positive Umod voltage applied to the sample. Visualization of the peculiarity obtained is performed by means of scanning a larger area at the Uvis of the
Fig. 2. Image of the silicon surface modified under a positive voltage (morphology, three-dimensional presentation and profile). Modification voltage, 4.5 V; visualization voltage, 3.26 V. Modified area size 0.830.8 mm, 6–8 nm deep.
V.M. Kornilov, A.N. Lachinov / Microelectronic Engineering 69 (2003) 399–404
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Fig. 3. Image of the silicon surface modified under a negative voltage (morphology, three-dimensional presentation and profile). Modification voltage, 24.5 V; visualization voltage, 23.34 V. Modified area sizes 0.230.2, 0.530.5 and 131 mm; height 5–8 nm.
same polarity. If the same procedure is carried out in the reverse polarity (minus on the sample) the modified area is displayed as a protrusion. Fig. 3 presents an image of the Si surface with three separate modified areas. These areas were modified one after another with intermediate control of each mark as described above. It is interesting to note that the scanning parameters were almost the same for measurements made with different voltage polarities. However, an essential distinction was in the character of the modification obtained. Modified areas obtained in negative polarity look like structured protrusions, i.e. are composed of smaller elements (Fig. 3, profile). Those obtained in the positive polarity look like cavities and no structure is observed (Fig. 2, profile). To interpret the data obtained let us consider the energy structure of the multi-layer system: STMprobe–adsorbate layer–SiO 2 –Si (Fig. 4). The same structures are used as elements when fabricating nonvolatile memory, the operation principle of which is based on the capture of the injected charge in the insulator layer [6]. The charge captured in the insulator (adsorbate) layer can sufficiently affect the
tunneling parameters, which is reflected in the STM image. When a negative voltage is applied to an n-type semiconductor substrate, an enriched layer appears near the Si–SiO 2 interface. At high voltage, the charge carriers tunnel through the oxide layer following the Fouler–Nordheim law. This occurs when the voltage applied exceeds the height of the potential barrier on the Si–SiO 2 interface (3.2 V). The charge captured in the adsorbate layer can reduce the electron affinity and increase additional emission components of the current. The latter may be registered by the STM as a protrusion on the surface. When a positive voltage is applied to an n-type semiconductor substrate, a depletion or even an inverse layer appears near the Si–SiO 2 interface. This may reduce the tunneling probability, which can be registered by the STM as a cavity. The suggested model of the modification of the interface charge state predicts the possibilities of writing, reading, erasing and rewriting information by means of a polarity change. All of these possibilities were realized in this work (Fig. 5). Rewriting of the information was performed as
402
V.M. Kornilov, A.N. Lachinov / Microelectronic Engineering 69 (2003) 399–404
Fig. 4. Energy diagrams of the experimental structure under different voltage U polarities [6]. Upper scheme corresponds to a negative voltage applied to the sample, and the lower to a positive voltage applied to the sample. Arrows indicate charge carrier movement. EF , Fermi level.
follows. First, the image of the modified surface under a negative voltage was obtained (square mark in Fig. 5a). This area was then scanned under a positive voltage applied to the sample (Fig. 5b). After the scanning, the mark faded away. Subsequent scanning under a negative voltage revealed no mark (Fig. 5c). Fig. 5d presents a new mark written at the location of the previous one. The electrochemical processes of depassivation or oxidation alone can hardly explain Si surface modification during STM investigations. A structure of the insulator–oxide–semiconductor type forms on
the investigated surface with the adsorbate layer as the insulator. Injected through the oxide layer, charge carriers are captured in the insulator layer. The STM methods allow one to stimulate the injection process and to register the resulting redistribution of the charge near the Si–SiO 2 surface.
3. The Si–SiO 2 –polymer system: surface modification Usually, the Si surface modification obtained
Fig. 5. Illustration of the possibility of information rewriting on the same area of the sample. The dashed line outlines the modified area of the Si surface. (a) Modified area; modification voltage, 24.5 V; visualization voltage, 23.14 V. (b) The same area; visualization voltage, 13.3 V. (c) The same area; visualization voltage, 23.14 V. (d) New mark created at the location of the previous one under the same conditions.
V.M. Kornilov, A.N. Lachinov / Microelectronic Engineering 69 (2003) 399–404
during STM measurements is explained by irreversible oxidation [1–4,7,8]. The adsorbate molecular layer is considered to assist with the oxidation. However, different functions are attributed to this layer in different reports. Thus the role of this layer in the formation of structures on the surface of the Si–SiO 2 system is not clear. In this work a thin polymer film was used to exclude the adsorption– desorption processes on the Si surface. It was shown previously that a poly(heteroarylene) film up to 100 nm thick deposited on a conductive surface can be investigated by the STM method [9]. It is well known that the processes at the metal–polymer interface significantly affect the electronic properties of thin polymer films, particularly by influencing the characteristics of the potential barrier at the interface. This makes it possible to use such films as unique charge sensors [10,11]. The polymer film was obtained from a solution in cyclohexanone by centrifugation on a silicon surface. The thickness of the polymer films used in the experiments varied from 40 to 60 nm. Fig. 6 presents an image of a polymer surface after the preliminary scanning of four areas. The operation
403
order was as follows. Four small areas of the surface were initially scanned with a high applied voltage (modification mode, minus on the sample). Then a larger area containing the four smaller modified areas was scanned at a lower applied voltage (visualization mode). In the visualization mode a stable image can be obtained at 3 , Uvis , 6 V, 10 , I , 100 pA. These parameters are similar to those used for the investigation of the initial Si–SiO 2 surface [12]. If the same procedures were carried out with the polarity reversed (plus on the sample), the modified areas look like depressions (Fig. 7). The conditions for ‘‘rewriting’’ on the same location were determined. When the negative polarity is on an n-type semiconductor, the enriched layer forms at the semiconductor–polymer interface. Scanning under high voltage induces tunneling of the charge carriers through the oxide layer following the Fouler–Nordheim law. The carriers are partially captured on the polymer film traps. Another part of the carriers forms the tunneling current. The captured charge may lead to the formation of deep electron states and even a narrow subband near the middle of the polymer band gap.
Fig. 6. Image of four modified (under a negative voltage) areas of the surface of a polymer film deposited on silicon. Morphology, three-dimensional presentation and profile. Modifying voltage, 24.5 V; visualization voltage, 23.4 V. Area 0.530.5 mm, high relief 100–130 nm.
404
V.M. Kornilov, A.N. Lachinov / Microelectronic Engineering 69 (2003) 399–404
Fig. 7. Image of the modified (under a positive voltage) area of the surface of a polymer film deposited on silicon. Morphology and profile. Modifying voltage, 4.5 V; visualization voltage, 3.4 V.
When a positive voltage is applied to the sample, a depletion layer forms near the polymer interface, which may lead to a decrease in the tunneling probability between the probe and the sample. Thus it has been shown that the Si surface modification occurring during STM measurements is significantly affected by charge processes at the interfaces. The electron mechanism of the surface modification makes it possible to use STM for nanoscale writing, rewriting and reading information in Si–SiO 2 –polymer structures.
References [1] J.A. Dagata, J. Schneir, H.H. Harary et al., Appl. Phys. Lett. 56 (1990) 2001. [2] P. Fay, R.T. Brockenbrough, G.J. Abeln et al., J. Appl. Phys. 75 (1994) 7545.
[3] L.N. Bolotov, V.A. Kozlov, I.V. Makarenko, A.N. Titkov, Phys. Techn. Semicond. 27 (1993) 1375. [4] N. Barniol, F. Perez-Murano, G. Abadal, J.H. Ye, X. Aymerich, Microelectron. Eng. 27 (1995) 27. [5] S.Yu. Vassiliev, A.V. Denisov, Techn. Phys. 70 (2000) 100. [6] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. [7] J.W. Lyding, T.-C. Shen, J.S. Hubacek et al., Appl. Phys. Lett. 64 (1994) 2010. [8] G. Abadal, F. Perez-Murano, N. Barniol, X. Aymerich, Appl. Phys. A 66 (1998) 791. [9] V.C. Kornilov, A.N. Lachinov, Techn. Phys. Lett. 26 (2000) 37. [10] V.C. Kornilov, A.N. Lachinov, J. Exp. Theor. Phys. 84 (1997) 833. [11] A.N. Lachinov, T.G. Zagurenko, V.C. Kornilov et al., Phys. Solid State 42 (2000) 1935. [12] A.N. Lachinov, V.C. Kornilov, in: International Workshop ‘‘Scanning Probe Microscopy—2002’’. Book of Abstracts, Nizhni Novgorod, 2002.