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SiO2and Si nanoscale patterning with an atomic force microscope
Superlattices and Microstructures, Vol. 23, No. 2, 1998
SiO2 and Si nanoscale patterning with an atomic force microscope B. Klehn, U. Kunze Lehrstuhl f¨ur Werkstoffe der Elektrotechnik, Ruhr-Universit¨at Bochum, D-44780 Bochum, Germany (Received 15 July 1996) The use of an atomic force microscope (AFM) as a nanolithographic tool is demonstrated. A photoresist layer several nanometre thin is indented by the vibrating AFM tip, where software control switches the tapping force from the imaging to the patterning mode. The resist pattern is transferred into a 10 nm SiO2 layer on Si(100) by wet chemical etching resulting in 20–40 nm wide lines. Subsequent transfer into the Si substrate using anisotropic KOH etching formed 60 nm wide V grooves. c 1998 Academic Press Limited
Key words: nanolithographic, atomic force microscope.
1. Introduction Presently there is a large interest in lithographic techniques based on scanning probe microscopy (SPM) (for a review see, e.g., [1–4] and references therein). In spite of the slow scanning speed compared with electron or ion beam lithography the SPM technique is advantageous because it is free from the proximity effect or radiation-induced crystal damage. In semiconductor nanostructure fabrication the latter is particularly important because the low scattering rate in a high-mobility two-dimensional electron gas should be preserved as far as possible. Among the different SPM techniques we are most interested in those which are also applicable to insulating surface layers. Only recently a corresponding method has been reported which relies on the plastic deformation of a thin polymer layer by the repulsive force of the vibrating tip of an atomic force microscope (AFM) [5, 6]. Using this dynamical ploughing technique the fabrication of arrays of dots indented in a thin photoresist layer with a period as small as 9 nm has been demonstrated, and dot arrays with periods down to 33 nm could be transferred into the GaAs substrate by wet-chemical etching [7]. In the present work we have developed this technique into a lithographic tool that can be used to prepare arbitrarily shaped lines and dot arrays and which allows easy alignment with respect to prefabricated patterns.
2. Experiment The Si(100) substrates were oxidized with 200 nm SiO2 . Except for 50×100 µm2 alignment marks the oxide was removed by buffered HF and another 10 nm SiO2 was thermally grown. Highly thinned photoresist (Shipley SP 2510) was spun on the wafer at 3500 rev min−1 followed by a 1-min bake at 95 ◦ C in air. A dilution of 1 part resist by 30 to 80 parts thinner resulted in a thickness ranging from 9.2 to 3.8 nm, respectively. Dipping the resist-covered sample in buffered HF revealed pinhole-free resist layers if the dilution was 1:50 or less. In the following experiments we used the ratio 1:50 and obtained a homogeneous and reproducible resist thickness of d = 5.4 ± 0.4 nm as measured by ellipsometry and AFM measurements. Ellipsometry on resist 0749–6036/98/020441 + 04 $25.00/0
sm960358
c 1998 Academic Press Limited
442
Superlattices and Microstructures, Vol. 23, No. 2, 1998
10
Height (nm)
a
b
c
d
e
f
g
h
5
0
0
200
400
600
Position (nm) Fig. 1. AFM image (top) of a dot pattern in resist created by AFM with different excitation amplitude. A cross section is shown at the bottom. The drive amplitude increases linearly from the point a (2.5 times the amplitude for imaging) to h (×8.3), the large arrow e indicates the optimum amplitude (×5.8) for lithography.
layers in the range 50 nm
Superlattices and Microstructures, Vol. 23, No. 2, 1998
443
A Resist
1500 0
1000 500
Height (nm)
10
0
500
1000 1500
5
0
(nm)
500 Position (nm)
1000
500 Position (nm)
1000
500 Position (nm)
1000
B SiO2
1500 0
1000
Height (nm)
20
500
0
500
1000 1500
10
0
(nm)
C Si
1500 0
1000 500 500
1000 1500
(nm)
Height (nm)
20 10 0
0
Fig. 2. 1.8 µm ×1.8 µm AFM image and sectional view of (A) a patterned resist layer, (B) the 10 nm SiO2 layer after pattern transfer,(C) the Si surface after V groove etching. The period is 250 nm. The real depth of the grooves in resist, SiO2 , and in Si is larger than revealed by the cross sections, because the tip is too thick.
more than 1 mm, at larger amplitude the linewidth increases indicating tip degradation. As the tip penetrates the resist the material is displaced usually to one side, as shown in Fig. 1 or, more clearly, in Fig. 2A. We assume that this asymmetry results from an irregularly shaped tip or from a lateral component in the cantilever oscillation. Figure 2 displays a section of a field of 40 lines of 2 µm length and 250 nm pitch. Due to the wall of displaced resist beside each of the trenches the minimum period of such a grating is about 40 nm. However, unlike the proximity effect in electron-beam lithography the wall does not cause a problem in forming X-
444
Superlattices and Microstructures, Vol. 23, No. 2, 1998
1500 0
1000 500 500
1000 1500
(nm)
Fig. 3. Topography of SiO2 surface after pattern transfer and removal of the resist. The linewidth at the surface is less than 40 nm.
or T-shaped structures. The resist pattern has been transferred into the oxide (Fig. 2B) by 45 s etching in buffered HF (etch rate 20 nm min−1 ) and further into the Si substrate by 10 s orientation-dependent etching in a solution of 50 wt% KOH and H2 O at 70 ◦ C (etch rate 290 nm min−1 ). Figure 2C shows an AFM image of the Si surface after stripping the oxide. Since the orientation of the lines is along [110] the anisotropic etching develops V grooves bounded by (111) planes. The top width of the grooves is 58 ± 2 nm, thus the depth of an ideal V groove is about 40 nm, which cannot be probed because of the 20–30 nm tip diameter. The apparent depth of the SiO2 grooves (Fig. 2B) is also limited by the tip diameter, therefore we may conclude that the bottom width is 20–30 nm or even smaller. Figure 3 gives an example of arbitrarily oriented line segments etched in SiO2 forming 300 nm size letters.
References [1] [2] [3] [4] [5]
}H. Rohrer, Microelectron. Eng. 27, 3 (1995). }O. T. Teuschler, K. Mahr, S. Miyazaki, and L. Ley, Appl. Phys. Lett. 66, 2499 (1995). }G. C. Abeln, T.-C. Shen, J. R. Tucker, and J. W. Lyding, Microelectron. Eng. 27, 23 (1995). }N. Kramer, J. Jorritsma, H. Birk, and C. Sch¨onenberger, Microelectron. Eng. 27, 47 (1995). }T. A. Jung, A. Moser, H. J. Hug, D. Brodbeck, R. Hofer, H. R. Hidber, and U. D. Schwarz, Ultramicroscopy 42–44, 1446 (1992). [6] }M. Wendel, S. K¨uhn, H. Lorenz, and J. P. Kotthaus, Appl. Phys. Lett. 65, 1775 (1994). [7] }M. Wendel, H. Lorenz, and J. P. Kotthaus, Appl. Phys. Lett. 67, 3732 (1995).
SiO2 and Si nanoscale patterning with an atomic force microscope B. Klehn, U. Kunze Lehrstuhl f¨ur Werkstoffe der Elektrotechnik, Ruhr-Universit¨at Bochum, D-44780 Bochum, Germany (Received 15 July 1996) The use of an atomic force microscope (AFM) as a nanolithographic tool is demonstrated. A photoresist layer several nanometre thin is indented by the vibrating AFM tip, where software control switches the tapping force from the imaging to the patterning mode. The resist pattern is transferred into a 10 nm SiO2 layer on Si(100) by wet chemical etching resulting in 20–40 nm wide lines. Subsequent transfer into the Si substrate using anisotropic KOH etching formed 60 nm wide V grooves. c 1998 Academic Press Limited
Key words: nanolithographic, atomic force microscope.
1. Introduction Presently there is a large interest in lithographic techniques based on scanning probe microscopy (SPM) (for a review see, e.g., [1–4] and references therein). In spite of the slow scanning speed compared with electron or ion beam lithography the SPM technique is advantageous because it is free from the proximity effect or radiation-induced crystal damage. In semiconductor nanostructure fabrication the latter is particularly important because the low scattering rate in a high-mobility two-dimensional electron gas should be preserved as far as possible. Among the different SPM techniques we are most interested in those which are also applicable to insulating surface layers. Only recently a corresponding method has been reported which relies on the plastic deformation of a thin polymer layer by the repulsive force of the vibrating tip of an atomic force microscope (AFM) [5, 6]. Using this dynamical ploughing technique the fabrication of arrays of dots indented in a thin photoresist layer with a period as small as 9 nm has been demonstrated, and dot arrays with periods down to 33 nm could be transferred into the GaAs substrate by wet-chemical etching [7]. In the present work we have developed this technique into a lithographic tool that can be used to prepare arbitrarily shaped lines and dot arrays and which allows easy alignment with respect to prefabricated patterns.
2. Experiment The Si(100) substrates were oxidized with 200 nm SiO2 . Except for 50×100 µm2 alignment marks the oxide was removed by buffered HF and another 10 nm SiO2 was thermally grown. Highly thinned photoresist (Shipley SP 2510) was spun on the wafer at 3500 rev min−1 followed by a 1-min bake at 95 ◦ C in air. A dilution of 1 part resist by 30 to 80 parts thinner resulted in a thickness ranging from 9.2 to 3.8 nm, respectively. Dipping the resist-covered sample in buffered HF revealed pinhole-free resist layers if the dilution was 1:50 or less. In the following experiments we used the ratio 1:50 and obtained a homogeneous and reproducible resist thickness of d = 5.4 ± 0.4 nm as measured by ellipsometry and AFM measurements. Ellipsometry on resist 0749–6036/98/020441 + 04 $25.00/0
sm960358
c 1998 Academic Press Limited
442
Superlattices and Microstructures, Vol. 23, No. 2, 1998
10
Height (nm)
a
b
c
d
e
f
g
h
5
0
0
200
400
600
Position (nm) Fig. 1. AFM image (top) of a dot pattern in resist created by AFM with different excitation amplitude. A cross section is shown at the bottom. The drive amplitude increases linearly from the point a (2.5 times the amplitude for imaging) to h (×8.3), the large arrow e indicates the optimum amplitude (×5.8) for lithography.
layers in the range 50 nm
Superlattices and Microstructures, Vol. 23, No. 2, 1998
443
A Resist
1500 0
1000 500
Height (nm)
10
0
500
1000 1500
5
0
(nm)
500 Position (nm)
1000
500 Position (nm)
1000
500 Position (nm)
1000
B SiO2
1500 0
1000
Height (nm)
20
500
0
500
1000 1500
10
0
(nm)
C Si
1500 0
1000 500 500
1000 1500
(nm)
Height (nm)
20 10 0
0
Fig. 2. 1.8 µm ×1.8 µm AFM image and sectional view of (A) a patterned resist layer, (B) the 10 nm SiO2 layer after pattern transfer,(C) the Si surface after V groove etching. The period is 250 nm. The real depth of the grooves in resist, SiO2 , and in Si is larger than revealed by the cross sections, because the tip is too thick.
more than 1 mm, at larger amplitude the linewidth increases indicating tip degradation. As the tip penetrates the resist the material is displaced usually to one side, as shown in Fig. 1 or, more clearly, in Fig. 2A. We assume that this asymmetry results from an irregularly shaped tip or from a lateral component in the cantilever oscillation. Figure 2 displays a section of a field of 40 lines of 2 µm length and 250 nm pitch. Due to the wall of displaced resist beside each of the trenches the minimum period of such a grating is about 40 nm. However, unlike the proximity effect in electron-beam lithography the wall does not cause a problem in forming X-
444
Superlattices and Microstructures, Vol. 23, No. 2, 1998
1500 0
1000 500 500
1000 1500
(nm)
Fig. 3. Topography of SiO2 surface after pattern transfer and removal of the resist. The linewidth at the surface is less than 40 nm.
or T-shaped structures. The resist pattern has been transferred into the oxide (Fig. 2B) by 45 s etching in buffered HF (etch rate 20 nm min−1 ) and further into the Si substrate by 10 s orientation-dependent etching in a solution of 50 wt% KOH and H2 O at 70 ◦ C (etch rate 290 nm min−1 ). Figure 2C shows an AFM image of the Si surface after stripping the oxide. Since the orientation of the lines is along [110] the anisotropic etching develops V grooves bounded by (111) planes. The top width of the grooves is 58 ± 2 nm, thus the depth of an ideal V groove is about 40 nm, which cannot be probed because of the 20–30 nm tip diameter. The apparent depth of the SiO2 grooves (Fig. 2B) is also limited by the tip diameter, therefore we may conclude that the bottom width is 20–30 nm or even smaller. Figure 3 gives an example of arbitrarily oriented line segments etched in SiO2 forming 300 nm size letters.
References [1] [2] [3] [4] [5]
}H. Rohrer, Microelectron. Eng. 27, 3 (1995). }O. T. Teuschler, K. Mahr, S. Miyazaki, and L. Ley, Appl. Phys. Lett. 66, 2499 (1995). }G. C. Abeln, T.-C. Shen, J. R. Tucker, and J. W. Lyding, Microelectron. Eng. 27, 23 (1995). }N. Kramer, J. Jorritsma, H. Birk, and C. Sch¨onenberger, Microelectron. Eng. 27, 47 (1995). }T. A. Jung, A. Moser, H. J. Hug, D. Brodbeck, R. Hofer, H. R. Hidber, and U. D. Schwarz, Ultramicroscopy 42–44, 1446 (1992). [6] }M. Wendel, S. K¨uhn, H. Lorenz, and J. P. Kotthaus, Appl. Phys. Lett. 65, 1775 (1994). [7] }M. Wendel, H. Lorenz, and J. P. Kotthaus, Appl. Phys. Lett. 67, 3732 (1995).