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Si(111) observed by an atomic force microscope
Surface Science 443 (1999) L1055–L1058 www.elsevier.nl/locate/susc
Surface Science Letters
Interface states of SiO /Si(111) observed by an 2 atomic force microscope Ryu Hasunuma a, *, Yasushiro Nishioka a, Atsushi Ando b, Kazushi Miki b a Texas Instruments Tsukuba R&D Center Limited, 17 Miyukigaoka, Tsukuba, Ibaraki 305-0841, Japan b Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan Received 30 June 1999; accepted for publication 21 September 1999
Abstract We investigated the distribution of tunneling current through ultrathin oxide films on Si(111) with an atomic force microscope having a conductive tip. We observed enhancement of the tunneling current at step edges for the oxide grown in dry O at 600°C, while the oxide grown in NHO showed only small contrast over the surface. With analysis 2 3 of the current–voltage characteristics, the tunneling current enhancement could reflect the interface states at the step edges. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy (AFM ); Semiconductor–insulator interfaces; Silicon oxides
One of the most important key techniques to realizing stable, ultrathin gate oxides for future ultra-large-scale integrated circuit ( ULSI ) devices is the precise control of oxide defects and interface states. These defects and traps are considered to be closely related to breakdown phenomena [1], and their non-uniform distribution could give rise to fluctuations in electrical properties, such as the threshold voltage in metal-oxide-semiconductor (MOS) devices. However, there is difficulty in analyzing the electrical properties of ultrathin oxides, less than 3 nm, basically because of large tunneling leakage. In addition, conventional methods such as capacitance–voltage (C–V ) measurements lack two-dimensional resolution. Atomic * Corresponding author. Tel.: +81-298-50-1733; fax: +81-298-50-1729. E-mail address: [email protected] (R. Hasunuma)
force microscopy (AFM ), with a fine probe as the top electrode, is one method that can be applied to investigate electrical properties, such as the breakdown characteristics, with nano-scale resolution [2]. However, there have been some difficulties in the analysis of interface states with AFM. This is because the bias voltage should be small to detect the interface states in the bandgap, which could give rise to an extremely weak current signal. In this paper we focused on the distribution of interface states in oxides grown on atomically flat Si(111) surfaces. The detection of a signal related to the interface states was achieved by using ultrathin oxides. We visualized for the first time the correlation between the interface state distribution and surface morphology, such as atomic steps. The ultrathin oxides were formed on p-type (boron-doped, 1–10 V cm) Si(111) wafers in two oxidation processes. One was in situ dry O oxida2
0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 10 2 3 -7
SU RF A LE CE TT S ER CIE S N CE
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R. Hasunuma et al. / Surface Science 443 (1999) L1055–L1058
tion (2×10−4 Pa at 600°C for 10 min) in an ultrahigh vacuum ( UHV ) chamber after flashing the samples at 1100°C to form the atomically clean and flat surfaces. The other was formation of native oxide by boiling NHO solution, after flat3 tening the surface in NH F solution. This oxide 4 proceeded to a thinning process in HF. The thickness of both oxides was estimated as less than 1 nm. Since these oxide surfaces maintain the flat step–terrace structures which appear initially in the flashing process or in the flattening process in NH F, we can reduce the effects of surface rough4 ness on the electrical analysis of the interfaces. AFM measurements were done in vacuum, base pressure less than 1×10−5 Pa, with a commercial system (Seiko Instruments, SPI-300HV ) in contact mode. The silicon cantilever was coated with platinum by electron-beam deposition to obtain tunneling current images. Prior to observation, the samples were pre-annealed at 150°C in the analysis chamber to remove adsorbed water from the oxide surfaces. This process was done to reduce the size of electrical contact between the tip and substrate, and also to avoid additional anodic oxidation, which could occur during AFM measurements under biases. Fig. 1a–c show a topographic AFM image of the oxide grown in dry O , a current image 2 obtained simultaneously with (a) and current versus voltage curves obtained at the points indicated in (b), respectively. It can be seen from the topographic image that the oxide surface maintains the flat step–terrace structure. The contrast on terraces in the current image results probably from the fluctuation of oxide thickness, although we cannot completely ignore other effects, such as fluctuation in contact size between the AFM tip and the surface, etc. One of the most noteworthy features is the relatively larger tunneling current around the step edges than on terraces, as seen in the current image. The current–voltage (I–V ) measurements also exhibited a larger current around the step edges, as seen in Fig. 1c, which is consistent with the contrast in current images. Further, the difference in tunneling current was mainly observed when |V |<1 V, while there was s not a great difference when |V |>1 V. Actually, we s
Fig. 1. (a) A 600 nm×600 nm topographic AFM image of the ultrathin oxide film on Si(111) grown in dry O . (b) A current 2 image obtained simultaneously with (a). (c) Current–voltage curves obtained at the points indicated in (b). The sample bias was kept at −1.0 V for (a) and (b). The contrasts are: (a) 0 to 17 nm and (b) −0.07 to 0.03 nA. The bright spots at the step edges in the topographic image are considered to be SiC contamination.
observed that the contrast was eliminated in the current images as we increased the bias. Now we discuss the origin of the current enhancement around the step edges. One of the possible reasons is a difference in oxide thickness at the step edges and on terraces. Green et al. calculated the values of tunneling current for oxides with various thicknesses on p-type substrates [3]. Their results showed that the thickness dependence on the tunneling current was elimi-
CE N IE SC RS CE E A TT RF LE SU
R. Hasunuma et al. / Surface Science 443 (1999) L1055–L1058
nated under large reverse bias for ultrathin oxide, which seems to explain our results well. This should occur when we use a metal with small work function as the gate electrode, aluminum for instance, on a p-type substrate. This is because the current flow is limited by the transportation of minority carriers through the substrate under large reverse bias when the barrier height is small, in which the tunneling rate is too high. However, this cannot explain our results completely, since we used a platinum tip with a larger work function. Further, we observed the elimination of current at large biases for both polarities. In fact, the I–V curves measured at points with different oxide thicknesses show completely distinct behaviors. Fig. 2a–c show a topographic image of the oxide with a thicker area at the center, a current image obtained simultaneously with (a) and current versus voltage curves obtained at the thicker and thinner points on the oxide, respectively. The bright square in the topographic image is the thicker oxide area, which was scanned before taking this image with the biased tip under a humid atmosphere. The oxide in this area was grown further and expanded in the depth direction by an anodic oxidation mechanism with the existence of a water layer between the tip and the substrate. Since the height difference between the bright area and the other area is about 0.1 nm, the oxide in the bright area can be considered to be thicker by 0.3–0.4 nm than the other area. As seen in Fig. 2c, the thickness difference of only 0.3– 0.4 nm had a significant influence on the current values even when |V |>1 V, which is an apparently s different manner from that in Fig. 1c. The most justifiable reason to explain the tunneling current enhancement around the step edges is the presence of interface states in the bandgap, which give rise to the variation of tunneling current only for small biases [4]. According to recent studies on oxide growth mechanisms [5–8], oxidation proceeds in a layer-by-layer manner with the random nucleation of oxidation, reflecting the initial surface morphology to the interface. This means that the steps at the interface should exist just under the steps on the oxide surface for the present experiments. Therefore, our results indicate that the formation of interface states occurred
L1057
Fig. 2. (a) A 600 nm×600 nm topographic AFM image of the ultrathin silicon oxide film. (b) A current image obtained simultaneously with (a). The sample bias was 1.0 V for (a) and (b). The contrasts are: (a) 0 to 3.2 nm and (b) 0.03 to 0.2 nA. The bright area in (a) was initially scanned a few times at V =2 V. (c) Current–voltage curves obtained at the thicker and s thinner points on the oxide.
preferentially around the steps at the interface. Of course, the configuration of interface states cannot be clarified by the AFM measurements. It should be noted, however, that the present results are consistent with those of Yasuda et al. who analyzed, by the C–V method, the relationship between initial surface morphology and interface state density for oxides grown by remote plasma [9]. The formation of interface states around the steps can be easily understood by relaxation of the compressive stress at the steps, which should accu-
SU RF A LE CE TT S ER CIE S N CE
L1058
R. Hasunuma et al. / Surface Science 443 (1999) L1055–L1058
ing current around step edges on the ultrathin silicon oxide grown in dry O . It was concluded 2 that the tunneling current was enhanced by carrier transportation via the interface. Although we still need further analysis for confirmation by comparison of the results with conventional methods, it was demonstrated that AFM has the potential for nano-scale analysis of the electrical properties of ultrathin oxides. Furthermore, these results could give us an insight into better control of SiO /Si 2 interfaces.
Acknowledgement This work was supported by NEDO.
References Fig. 3. (a) A 250 nm×250 nm topographic AFM image of the ultrathin oxide film on Si(111) grown in NHO . (b) A current 3 image obtained simultaneously with (a). The sample bias was kept at 1.0 V for (a) and (b). The contrasts are: (a) 0 to 0.6 nm and (b) 0 to 0.06 nA.
mulate during oxide growth due to volume expansion of the oxide. On the other hand, the oxide grown in NHO 3 solution did not exhibit the tunneling current enhancement around the step edges, as seen in Fig. 3. In this case, the steps did not act as preferential sites for interface state formation. The results could indicate that the relaxation behaviors are related the oxidation conditions, such as atmosphere, temperature, thickness and so on. In summary, we have observed a larger tunnel-
[1] T. Tanamoto, A. Toriumi, Jpn. J. Appl. Phys. 36 (1997) 1439. [2] S.J. O’Shea, R.M. Atta, M.P. Murrell, M.E. Welland, J. Vac. Sci. Technol. B 13 (1995) 1945. [3] M.A. Green, F.D. King, J. Shewchun, Solid-State Electronics 17 (1974) 551. [4] T. Sakoda, M. Matsumura, Y. Nishioka, Appl. Surf. Sci. 117/118 (1997) 241. [5] T. Komeda, K. Namba, Y. Nishioka, J. Vac. Sci. Technol. A 16 (1998) 1680. [6 ] H. Watanabe, K. Fujita, M. Ichikawa, Surf. Sci. 385 (1997) L952. [7] K. Ohishi, T. Hattori, Jpn. J. Appl. Phys. 33 (1994) L675. [8] T. Hattori, M. Fujimura, T. Yagi, M. Ohashi, Appl. Surf. Sci. 123/124 (1998) 87. [9] T. Yasuda, Y. Ma, G. Lucovsky, Extended Abstracts of the 1994 International Conference on Solid State Devices and Materials, Yokahama, (1994) 847.
Surface Science Letters
Interface states of SiO /Si(111) observed by an 2 atomic force microscope Ryu Hasunuma a, *, Yasushiro Nishioka a, Atsushi Ando b, Kazushi Miki b a Texas Instruments Tsukuba R&D Center Limited, 17 Miyukigaoka, Tsukuba, Ibaraki 305-0841, Japan b Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan Received 30 June 1999; accepted for publication 21 September 1999
Abstract We investigated the distribution of tunneling current through ultrathin oxide films on Si(111) with an atomic force microscope having a conductive tip. We observed enhancement of the tunneling current at step edges for the oxide grown in dry O at 600°C, while the oxide grown in NHO showed only small contrast over the surface. With analysis 2 3 of the current–voltage characteristics, the tunneling current enhancement could reflect the interface states at the step edges. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy (AFM ); Semiconductor–insulator interfaces; Silicon oxides
One of the most important key techniques to realizing stable, ultrathin gate oxides for future ultra-large-scale integrated circuit ( ULSI ) devices is the precise control of oxide defects and interface states. These defects and traps are considered to be closely related to breakdown phenomena [1], and their non-uniform distribution could give rise to fluctuations in electrical properties, such as the threshold voltage in metal-oxide-semiconductor (MOS) devices. However, there is difficulty in analyzing the electrical properties of ultrathin oxides, less than 3 nm, basically because of large tunneling leakage. In addition, conventional methods such as capacitance–voltage (C–V ) measurements lack two-dimensional resolution. Atomic * Corresponding author. Tel.: +81-298-50-1733; fax: +81-298-50-1729. E-mail address: [email protected] (R. Hasunuma)
force microscopy (AFM ), with a fine probe as the top electrode, is one method that can be applied to investigate electrical properties, such as the breakdown characteristics, with nano-scale resolution [2]. However, there have been some difficulties in the analysis of interface states with AFM. This is because the bias voltage should be small to detect the interface states in the bandgap, which could give rise to an extremely weak current signal. In this paper we focused on the distribution of interface states in oxides grown on atomically flat Si(111) surfaces. The detection of a signal related to the interface states was achieved by using ultrathin oxides. We visualized for the first time the correlation between the interface state distribution and surface morphology, such as atomic steps. The ultrathin oxides were formed on p-type (boron-doped, 1–10 V cm) Si(111) wafers in two oxidation processes. One was in situ dry O oxida2
0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 10 2 3 -7
SU RF A LE CE TT S ER CIE S N CE
L1056
R. Hasunuma et al. / Surface Science 443 (1999) L1055–L1058
tion (2×10−4 Pa at 600°C for 10 min) in an ultrahigh vacuum ( UHV ) chamber after flashing the samples at 1100°C to form the atomically clean and flat surfaces. The other was formation of native oxide by boiling NHO solution, after flat3 tening the surface in NH F solution. This oxide 4 proceeded to a thinning process in HF. The thickness of both oxides was estimated as less than 1 nm. Since these oxide surfaces maintain the flat step–terrace structures which appear initially in the flashing process or in the flattening process in NH F, we can reduce the effects of surface rough4 ness on the electrical analysis of the interfaces. AFM measurements were done in vacuum, base pressure less than 1×10−5 Pa, with a commercial system (Seiko Instruments, SPI-300HV ) in contact mode. The silicon cantilever was coated with platinum by electron-beam deposition to obtain tunneling current images. Prior to observation, the samples were pre-annealed at 150°C in the analysis chamber to remove adsorbed water from the oxide surfaces. This process was done to reduce the size of electrical contact between the tip and substrate, and also to avoid additional anodic oxidation, which could occur during AFM measurements under biases. Fig. 1a–c show a topographic AFM image of the oxide grown in dry O , a current image 2 obtained simultaneously with (a) and current versus voltage curves obtained at the points indicated in (b), respectively. It can be seen from the topographic image that the oxide surface maintains the flat step–terrace structure. The contrast on terraces in the current image results probably from the fluctuation of oxide thickness, although we cannot completely ignore other effects, such as fluctuation in contact size between the AFM tip and the surface, etc. One of the most noteworthy features is the relatively larger tunneling current around the step edges than on terraces, as seen in the current image. The current–voltage (I–V ) measurements also exhibited a larger current around the step edges, as seen in Fig. 1c, which is consistent with the contrast in current images. Further, the difference in tunneling current was mainly observed when |V |<1 V, while there was s not a great difference when |V |>1 V. Actually, we s
Fig. 1. (a) A 600 nm×600 nm topographic AFM image of the ultrathin oxide film on Si(111) grown in dry O . (b) A current 2 image obtained simultaneously with (a). (c) Current–voltage curves obtained at the points indicated in (b). The sample bias was kept at −1.0 V for (a) and (b). The contrasts are: (a) 0 to 17 nm and (b) −0.07 to 0.03 nA. The bright spots at the step edges in the topographic image are considered to be SiC contamination.
observed that the contrast was eliminated in the current images as we increased the bias. Now we discuss the origin of the current enhancement around the step edges. One of the possible reasons is a difference in oxide thickness at the step edges and on terraces. Green et al. calculated the values of tunneling current for oxides with various thicknesses on p-type substrates [3]. Their results showed that the thickness dependence on the tunneling current was elimi-
CE N IE SC RS CE E A TT RF LE SU
R. Hasunuma et al. / Surface Science 443 (1999) L1055–L1058
nated under large reverse bias for ultrathin oxide, which seems to explain our results well. This should occur when we use a metal with small work function as the gate electrode, aluminum for instance, on a p-type substrate. This is because the current flow is limited by the transportation of minority carriers through the substrate under large reverse bias when the barrier height is small, in which the tunneling rate is too high. However, this cannot explain our results completely, since we used a platinum tip with a larger work function. Further, we observed the elimination of current at large biases for both polarities. In fact, the I–V curves measured at points with different oxide thicknesses show completely distinct behaviors. Fig. 2a–c show a topographic image of the oxide with a thicker area at the center, a current image obtained simultaneously with (a) and current versus voltage curves obtained at the thicker and thinner points on the oxide, respectively. The bright square in the topographic image is the thicker oxide area, which was scanned before taking this image with the biased tip under a humid atmosphere. The oxide in this area was grown further and expanded in the depth direction by an anodic oxidation mechanism with the existence of a water layer between the tip and the substrate. Since the height difference between the bright area and the other area is about 0.1 nm, the oxide in the bright area can be considered to be thicker by 0.3–0.4 nm than the other area. As seen in Fig. 2c, the thickness difference of only 0.3– 0.4 nm had a significant influence on the current values even when |V |>1 V, which is an apparently s different manner from that in Fig. 1c. The most justifiable reason to explain the tunneling current enhancement around the step edges is the presence of interface states in the bandgap, which give rise to the variation of tunneling current only for small biases [4]. According to recent studies on oxide growth mechanisms [5–8], oxidation proceeds in a layer-by-layer manner with the random nucleation of oxidation, reflecting the initial surface morphology to the interface. This means that the steps at the interface should exist just under the steps on the oxide surface for the present experiments. Therefore, our results indicate that the formation of interface states occurred
L1057
Fig. 2. (a) A 600 nm×600 nm topographic AFM image of the ultrathin silicon oxide film. (b) A current image obtained simultaneously with (a). The sample bias was 1.0 V for (a) and (b). The contrasts are: (a) 0 to 3.2 nm and (b) 0.03 to 0.2 nA. The bright area in (a) was initially scanned a few times at V =2 V. (c) Current–voltage curves obtained at the thicker and s thinner points on the oxide.
preferentially around the steps at the interface. Of course, the configuration of interface states cannot be clarified by the AFM measurements. It should be noted, however, that the present results are consistent with those of Yasuda et al. who analyzed, by the C–V method, the relationship between initial surface morphology and interface state density for oxides grown by remote plasma [9]. The formation of interface states around the steps can be easily understood by relaxation of the compressive stress at the steps, which should accu-
SU RF A LE CE TT S ER CIE S N CE
L1058
R. Hasunuma et al. / Surface Science 443 (1999) L1055–L1058
ing current around step edges on the ultrathin silicon oxide grown in dry O . It was concluded 2 that the tunneling current was enhanced by carrier transportation via the interface. Although we still need further analysis for confirmation by comparison of the results with conventional methods, it was demonstrated that AFM has the potential for nano-scale analysis of the electrical properties of ultrathin oxides. Furthermore, these results could give us an insight into better control of SiO /Si 2 interfaces.
Acknowledgement This work was supported by NEDO.
References Fig. 3. (a) A 250 nm×250 nm topographic AFM image of the ultrathin oxide film on Si(111) grown in NHO . (b) A current 3 image obtained simultaneously with (a). The sample bias was kept at 1.0 V for (a) and (b). The contrasts are: (a) 0 to 0.6 nm and (b) 0 to 0.06 nA.
mulate during oxide growth due to volume expansion of the oxide. On the other hand, the oxide grown in NHO 3 solution did not exhibit the tunneling current enhancement around the step edges, as seen in Fig. 3. In this case, the steps did not act as preferential sites for interface state formation. The results could indicate that the relaxation behaviors are related the oxidation conditions, such as atmosphere, temperature, thickness and so on. In summary, we have observed a larger tunnel-
[1] T. Tanamoto, A. Toriumi, Jpn. J. Appl. Phys. 36 (1997) 1439. [2] S.J. O’Shea, R.M. Atta, M.P. Murrell, M.E. Welland, J. Vac. Sci. Technol. B 13 (1995) 1945. [3] M.A. Green, F.D. King, J. Shewchun, Solid-State Electronics 17 (1974) 551. [4] T. Sakoda, M. Matsumura, Y. Nishioka, Appl. Surf. Sci. 117/118 (1997) 241. [5] T. Komeda, K. Namba, Y. Nishioka, J. Vac. Sci. Technol. A 16 (1998) 1680. [6 ] H. Watanabe, K. Fujita, M. Ichikawa, Surf. Sci. 385 (1997) L952. [7] K. Ohishi, T. Hattori, Jpn. J. Appl. Phys. 33 (1994) L675. [8] T. Hattori, M. Fujimura, T. Yagi, M. Ohashi, Appl. Surf. Sci. 123/124 (1998) 87. [9] T. Yasuda, Y. Ma, G. Lucovsky, Extended Abstracts of the 1994 International Conference on Solid State Devices and Materials, Yokahama, (1994) 847.