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Combined AFM and laser lithography on hydrogen-passivated amorphous silicon
Superlattices and Microstructures, Vol. 20, No. 4, 1996
Combined AFM and laser lithography on hydrogen-passivated amorphous silicon ¨ llenborn, Karen Birkelund, Matthias Mu Franc ¸ ois Grey, Flemming Jensen Mikroelektronik Centret, Technical University of Denmark, Bldg. 345 east, DK-2800 Lyngby, Denmark
Steen Madsen DME–Danish Micro Engineering A/S, DK-2730 Herlev, Denmark
(Received 23 May 1996) We report a novel combination of AFM lithography and laser direct writing on hydrogenpassivated amorphous silicon surfaces to fabricate combined silicon milli-, micro- and nanostructures. Selective oxidation is performed by focusing a laser beam (λ = 458 nm) on a hydrogen-terminated silicon surface, forming the millimetre-size contact pads for connection of nanometre-scale patterns. The nanostructures are made by electric-fieldenhanced oxidation using a contact mode AFM equipped with a metal-coated tip. Both techniques are based on selective oxidation of hydrogen-passivated amorphous silicon, where the oxide is used as an etch mask in a single etch step. The lithographic process has also been demonstrated using a reflection mode scanning near-field optical microscope with an uncoated fiber probe. c 1996 Academic Press Limited
Key words: nanolithography, hydrogen passivated silicon, scanning probe microscopes.
1. Introduction The last few years have witnessed major efforts to develop atomic force microscopes (AFM) and scanning tunneling microscopes (STM) for lithographic purposes. Nanometre size structures have been made successfully in a variety of ways, for example by exposing a resist using the tip as a low energy electron source [1], or by scratching a resist with the tip [2, 3]. Alternative resist materials have been developed for scanning probe lithography, such as silicon and thin chromium or titanium films, all of which are sensitive to tip-induced oxidation processes [4–6]. Structures with dimensions down to 10 nm have been achieved using AFM lithography [7] and even smaller dimensions are possible by STM lithography [8]. More recently, efforts have been made to integrate STM and AFM lithography with more conventional lithographic tools for electrical device fabrication. Single electron devices have been fabricated using a STM [9], and the gate of a MOSFET was patterned by AFM [10]. For practical applications of nanostructures, it is essential to connect these structures with the macroscopic world, for example through larger contact pads. AFM and STM lithography are not suited for large area patterning because the scan range is typically less than 100 × 100 µm2 and the writing speed is too slow. 0749–6036/96/080555 + 06 $25.00/0
c 1996 Academic Press Limited
556
Superlattices and Microstructures, Vol. 20, No. 4, 1996
Another limitation is that the wear of the AFM tip during lithography is often considerable when covering larger areas. In this paper, we present a new combination of lithographic techniques, which is simple and powerful for fabrication of nanometre-size devices for electronic applications. We use a laser direct write system [11] and an AFM to locally oxidize a hydrogen-passivated amorphous silicon surface. Kramer et al. [12] have recently achieved linewidths of 200 nm on hydrogen-passivated crystalline silicon by letting two interfering UV-laser beams (λ = 350.7 nm) illuminate the surface for 15 min with a power of 150 mW. By using laser direct writing for local oxidation of a hydrogen-passivated amorphous silicon surface, we have achieved linewidths down to 500 nm with writing speeds up to 100 mm s−1 [13]. The laser writing is used for the larger patterning of contact pads and coarse (µm-scale) wiring while the AFM is used for making the nanostructures. The patterning of nanometre size structures using an AFM equipped with a conductive cantilever has been demonstrated by several groups [7, 14, 15], and linewidths below 50 nm have been achieved.
2. Method The patterning was performed on two different types of samples. One type consists of 500 nm thermal oxide grown on a n-type (100) silicon wafer covered with 100 nm amorphous boron-doped silicon deposited by direct current magnetron sputtering. The other sample has an oxide thickness of 100 nm and 13 nm amorphous silicon. Chips of 5 × 5 mm2 are passivated for 60 s in 5% diluted hydrofluoric acid (HF) and blow-dried with N2 before being placed on an x–y stage for the laser writing. The laser direct-write system used consists of a continuous-wave argon ion laser with UV option, beam conditioning and focusing optics, and high resolution direct current motor stages with internal encoders for sample translation. A computer controlled acousto-optic modulator switches the laser beam on and off during writing. The laser beam running at a wavelength of 458 nm is focused to a 0.5 µm spot on the sample surface. For selective oxidation of the sample with the thin amorphous silicon layer, a light power of 50 mW is used, while 200 mW are used for patterning the sample with the thick amorphous silicon layer. In both cases the writing speed is 5 mm s−1 . Detailed investigations of the mechanism involved in the laser oxidation process are discussed in Ref. [13]. The laser process defines the large structures and coarse wiring as a thin layer of oxide on the silicon surface. The laser-processed area can be identified optically for coarse alignment in the AFM. To carry out the finer lithographic step, the sample is mounted in a commercial DME Rasterscope 4000 AFM, and the area to be modified is placed under the tip. The AFM writing is performed in contact mode in air, scanning with a force of 0.8–2.0 nN and a scanning speed of 20 µm s−1 . The tip used for the selective oxidation is a standard Si3 N4 AFM cantilever (k = 0.58 N m−1 ) coated with 15 nm Ti. The hydrogen desorption and oxidation are performed by applying a voltage between −5 V and −10 V to the tip. The oxide thickness measured from a topological AFM-image increases with increasing applied negative voltage and are typically in the range 0.5–5.0 nm. After the oxidation, the sample is etched in 28 wt% potassium hydroxide (KOH) at room temperature, giving an etch rate of ∼ 20 nm min−1 . The oxide serves as an etch mask and the surrounding unprotected amorphous silicon is etched all the way to the thermal oxide. Figure 1A shows the resulting structure. The pads have an area of 0.5 × 0.24 mm2 , the wire is 2 µm wide and 300 µm long with a 3 µm wide gap in the center where a thinner line has been written with the AFM. An AFM image of the center part is shown in Fig. 1B. The height of the structure is 100 nm and the width of the AFM-written line is 320 nm. This line has been written by applying −8 V to the AFM tip over 20 parallel line scans. Figure 2 shows the narrowest wire obtained in this setup, which is 50 nm wide. The lithographic resolution appears to be limited by the tip shape.
3. Wear of the tip The AFM tip is rapidly worn down during the writing process, probably due to electrostatic forces. When
557
0.24 mm
Superlattices and Microstructures, Vol. 20, No. 4, 1996
0.5 mm
A
Height (nm)
140
00
420
nm
42
0 nm
120 100 80 60 40 320 nm
20 0 0
200
400
600
800
1000
nm
B
Fig. 1. A, An optical micrograph of a complete laser and AFM-defined structure after KOH-etching. The laser-written contact pads are 0.5 × 0.24 mm2 , the connecting wire is 300 × 2 µm2 . In a 3 µm gap in the connecting wire, a finer wire is written using AFM lithography. B, AFM-image of the center part showing a 320 nm wide, 100 nm high AFM-written wire connecting the 2 µm wide laser-written leads.
175
0 nm
17
5
m 0n
Height (nm)
12 8 4 0
0
100
200
300
400 nm
500
Fig. 2. A grid pattern fabricated with the AFM. After etching the lines are 50 nm wide and 10 nm high.
600
700
558
Superlattices and Microstructures, Vol. 20, No. 4, 1996 B
Normal scanning gh
t
A
5 4
*
* *
3
* *
2 1 0
ght
Applying a voltage
Change in force (nN)
La se
r li
6
La se
r li
–10
*
*
* *
–4 –2 –8 –6 Applied voltage (V)
0
– – – – – – + + + + + +
Fig. 3. A, A schematic of the bending of the cantilever without and with an applied voltage. B, The graph shows the dependence of tip displacement on applied voltage. The solid curve is a fit using the expression in the text.
a voltage is applied to the tip while scanning, electrostatic forces attract the tip and cantilever towards the scanned surface. This attraction causes a bending of the cantilever, resulting in a change of the deflected laser beam in such a way that the feedback system of the AFM responds as if an additional repulsive force is present. In Fig. 3A a sketch shows the bending of the cantilever with and without applying a voltage. The graph in Fig. 3B shows the changes in repulsive force as a function of applied voltage. The changes in repulsive force are calculated from the measured difference in the deflection of the cantilever with and without an applied voltage divided by the force constant of the cantilever. The points are an average of ten measurements. There is no difference whether a negative or positive voltage is applied, which agrees with the notion that the electrostatic forces are always attractive. Further, Fig. 3B shows a nonlinear dependence of the applied voltage which can be modeled simply in terms of a capacitor with area A, plate separation d, applied voltage V , and constant of vacuum permittivity ε such that the electrostatic force between the plates is Fe =
ε AV 2 . 2d 2
The continuous line in the figure is for d = 15 nm and A = 542 nm2 in reasonable agreement with the dimensions of the tip apex. To model this more accurately requires a full calculation of the electrostatic forces for the complex geometry of tip, cantilever and surface. The simple formula quoted in the text suggests that the force is produced mainly between tip apex and surface. The result of the electrostatic forces are a considerable wear of the coated tip. We are investigating ways to compensate for the electrostatic attraction, to increase the lifetime of the tips.
4. SNOM lithography Based on the results obtained with the laser direct writing, we have also used a scanning near-field optical microscope (SNOM) for surface illumination [16]. By operating in the optical near-field the SNOM overcomes
Superlattices and Microstructures, Vol. 20, No. 4, 1996
10.4 nm
11.3 nm 3500 nm
1050 nm
B
3500 nm
A
559
1050 nm
Fig. 4. AFM images after etching of lines written using the SNOM. A, Lines written with light coupled into the uncoated probe. B, 50 nm wide lines resulting from scanning without light coupled into the fiber.
the diffraction limit of conventional optical microscopy. Figure 4A shows a part of a grating consisting of 32 parallel lines after etching in KOH. The lines are written using the SNOM with a scan speed of 80 µm s−1 on hydrogen-passivated amorphous silicon. The surface is locally illuminated through an uncoated tapered fiber probe using an argon ion laser running at a wavelength of 458 nm. The far-field power emitted from the SNOM-probe is measured to be 25 mW. This again leads to a local oxidation of the silicon. Figure 4A shows a part of the total exposed area. Each scan line is composed of three lines, a center line of 110 nm and two broader lines of 240 nm on each side. The narrow line is believed to originate from the near-field light exposure, whereas the broader side lines result from interference with the optical far-field which penetrates the sidewalls of the uncoated probe [16]. Unexpectedly, linewidths of 50 nm have been obtained without light coupled into the probe. The hydrogen desorption may in this case be caused by an electrostatic potential between the probe and the amorphous silicon layer which is electrically isolated from the Si(100) substrate. Thus the SNOM appears to combine both methods of local oxidation, light-induced and field-induced, which are achieved separately by laser direct writing and AFM. One of the advantages of using the SNOM for lithography is that the wear of the tip is avoided, since the probe-to-sample distance is controlled by a shear-force microscope [17].
5. Conclusion In conclusion we have presented a novel combination of direct laser and AFM lithography for connecting nanometre size structures with micrometre and millimetre-scale circuitry. The technique offers a simple approach to patterning amorphous silicon surfaces by local oxidation of silicon films and subsequent etching.
References [1] L. A. Nagahara, P. I. Oden, A. Majumdar, J. P. Carrejo, J. Graham and K. Alexander, SPIE Scanning Probe Microscopies 1639, 171 (1992). [2] L. L. Sohn and R. L. Willet, NT2 FRIDAY, p. 1–4. [3] M. Wendel, S. K¨uhn, H. Lorenz and J. P. Kotthaus, Appl. Phys. Lett. 65, 1775 (1994).
560 [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Superlattices and Microstructures, Vol. 20, No. 4, 1996 E. S. Snow and P. M. Campbell, Appl. Phys. Lett. 64, 1932 (1994). D. Wang, L. Tsau and K. L. Wang, Appl. Phys. Lett. 67, 1295 (1995). S. C. Minne, Ph. Flueckiger, H. T. Soh and C. F. Quate, J. Vac. Sci. Technol. B 13, 1380 (1995). E. S. Snow, W. H. Juan, S. W. Pang and P. M. Campbell, Appl. Phys. Lett. 66, 1727 (1995). G. C. Abeln, T. C. Shen, J. R. Tucker and J. W. Lyding, Microelectronic Engineering 27, 23 (1995). K. Matsumoto, M. Ishii, K. Segawa and Y. Oka, Appl. Phys. Lett. 65, 34 (1996). S. C. Minne, H. T. Soh, Ph. Flueckiger and C. F. Quate, Appl. Phys. Lett. 66, 703 (1995). M. M¨ullenborn, H. Dirac J. W. Petersen, Appl. Phys. Lett. 66, 3001 (1995). N. Kramer, M. Niesten and C. Sch¨onenberger, Appl. Phys. Lett. 67, 2989 (1995). M. M¨ullenborn, K. Birkelund, S. Madsen and F. Grey, Appl. Phys. Lett., to be published. L. Tsau, D. Wang and K. L. Wang, Appl. Phys. Lett. 64, 2133 (1994). H. C. Day and D. R. Allee, Appl. Phys. Lett. 62, 2691 (1993). S. Madsen, M. M¨ullenborn, K. Birkelund and F. Grey, Appl. Phys. Lett., to be published. E. Betzig, P. L. Finn and I. S. Weiner, Appl. Phys. Lett. 60, 2484 (1992).
Combined AFM and laser lithography on hydrogen-passivated amorphous silicon ¨ llenborn, Karen Birkelund, Matthias Mu Franc ¸ ois Grey, Flemming Jensen Mikroelektronik Centret, Technical University of Denmark, Bldg. 345 east, DK-2800 Lyngby, Denmark
Steen Madsen DME–Danish Micro Engineering A/S, DK-2730 Herlev, Denmark
(Received 23 May 1996) We report a novel combination of AFM lithography and laser direct writing on hydrogenpassivated amorphous silicon surfaces to fabricate combined silicon milli-, micro- and nanostructures. Selective oxidation is performed by focusing a laser beam (λ = 458 nm) on a hydrogen-terminated silicon surface, forming the millimetre-size contact pads for connection of nanometre-scale patterns. The nanostructures are made by electric-fieldenhanced oxidation using a contact mode AFM equipped with a metal-coated tip. Both techniques are based on selective oxidation of hydrogen-passivated amorphous silicon, where the oxide is used as an etch mask in a single etch step. The lithographic process has also been demonstrated using a reflection mode scanning near-field optical microscope with an uncoated fiber probe. c 1996 Academic Press Limited
Key words: nanolithography, hydrogen passivated silicon, scanning probe microscopes.
1. Introduction The last few years have witnessed major efforts to develop atomic force microscopes (AFM) and scanning tunneling microscopes (STM) for lithographic purposes. Nanometre size structures have been made successfully in a variety of ways, for example by exposing a resist using the tip as a low energy electron source [1], or by scratching a resist with the tip [2, 3]. Alternative resist materials have been developed for scanning probe lithography, such as silicon and thin chromium or titanium films, all of which are sensitive to tip-induced oxidation processes [4–6]. Structures with dimensions down to 10 nm have been achieved using AFM lithography [7] and even smaller dimensions are possible by STM lithography [8]. More recently, efforts have been made to integrate STM and AFM lithography with more conventional lithographic tools for electrical device fabrication. Single electron devices have been fabricated using a STM [9], and the gate of a MOSFET was patterned by AFM [10]. For practical applications of nanostructures, it is essential to connect these structures with the macroscopic world, for example through larger contact pads. AFM and STM lithography are not suited for large area patterning because the scan range is typically less than 100 × 100 µm2 and the writing speed is too slow. 0749–6036/96/080555 + 06 $25.00/0
c 1996 Academic Press Limited
556
Superlattices and Microstructures, Vol. 20, No. 4, 1996
Another limitation is that the wear of the AFM tip during lithography is often considerable when covering larger areas. In this paper, we present a new combination of lithographic techniques, which is simple and powerful for fabrication of nanometre-size devices for electronic applications. We use a laser direct write system [11] and an AFM to locally oxidize a hydrogen-passivated amorphous silicon surface. Kramer et al. [12] have recently achieved linewidths of 200 nm on hydrogen-passivated crystalline silicon by letting two interfering UV-laser beams (λ = 350.7 nm) illuminate the surface for 15 min with a power of 150 mW. By using laser direct writing for local oxidation of a hydrogen-passivated amorphous silicon surface, we have achieved linewidths down to 500 nm with writing speeds up to 100 mm s−1 [13]. The laser writing is used for the larger patterning of contact pads and coarse (µm-scale) wiring while the AFM is used for making the nanostructures. The patterning of nanometre size structures using an AFM equipped with a conductive cantilever has been demonstrated by several groups [7, 14, 15], and linewidths below 50 nm have been achieved.
2. Method The patterning was performed on two different types of samples. One type consists of 500 nm thermal oxide grown on a n-type (100) silicon wafer covered with 100 nm amorphous boron-doped silicon deposited by direct current magnetron sputtering. The other sample has an oxide thickness of 100 nm and 13 nm amorphous silicon. Chips of 5 × 5 mm2 are passivated for 60 s in 5% diluted hydrofluoric acid (HF) and blow-dried with N2 before being placed on an x–y stage for the laser writing. The laser direct-write system used consists of a continuous-wave argon ion laser with UV option, beam conditioning and focusing optics, and high resolution direct current motor stages with internal encoders for sample translation. A computer controlled acousto-optic modulator switches the laser beam on and off during writing. The laser beam running at a wavelength of 458 nm is focused to a 0.5 µm spot on the sample surface. For selective oxidation of the sample with the thin amorphous silicon layer, a light power of 50 mW is used, while 200 mW are used for patterning the sample with the thick amorphous silicon layer. In both cases the writing speed is 5 mm s−1 . Detailed investigations of the mechanism involved in the laser oxidation process are discussed in Ref. [13]. The laser process defines the large structures and coarse wiring as a thin layer of oxide on the silicon surface. The laser-processed area can be identified optically for coarse alignment in the AFM. To carry out the finer lithographic step, the sample is mounted in a commercial DME Rasterscope 4000 AFM, and the area to be modified is placed under the tip. The AFM writing is performed in contact mode in air, scanning with a force of 0.8–2.0 nN and a scanning speed of 20 µm s−1 . The tip used for the selective oxidation is a standard Si3 N4 AFM cantilever (k = 0.58 N m−1 ) coated with 15 nm Ti. The hydrogen desorption and oxidation are performed by applying a voltage between −5 V and −10 V to the tip. The oxide thickness measured from a topological AFM-image increases with increasing applied negative voltage and are typically in the range 0.5–5.0 nm. After the oxidation, the sample is etched in 28 wt% potassium hydroxide (KOH) at room temperature, giving an etch rate of ∼ 20 nm min−1 . The oxide serves as an etch mask and the surrounding unprotected amorphous silicon is etched all the way to the thermal oxide. Figure 1A shows the resulting structure. The pads have an area of 0.5 × 0.24 mm2 , the wire is 2 µm wide and 300 µm long with a 3 µm wide gap in the center where a thinner line has been written with the AFM. An AFM image of the center part is shown in Fig. 1B. The height of the structure is 100 nm and the width of the AFM-written line is 320 nm. This line has been written by applying −8 V to the AFM tip over 20 parallel line scans. Figure 2 shows the narrowest wire obtained in this setup, which is 50 nm wide. The lithographic resolution appears to be limited by the tip shape.
3. Wear of the tip The AFM tip is rapidly worn down during the writing process, probably due to electrostatic forces. When
557
0.24 mm
Superlattices and Microstructures, Vol. 20, No. 4, 1996
0.5 mm
A
Height (nm)
140
00
420
nm
42
0 nm
120 100 80 60 40 320 nm
20 0 0
200
400
600
800
1000
nm
B
Fig. 1. A, An optical micrograph of a complete laser and AFM-defined structure after KOH-etching. The laser-written contact pads are 0.5 × 0.24 mm2 , the connecting wire is 300 × 2 µm2 . In a 3 µm gap in the connecting wire, a finer wire is written using AFM lithography. B, AFM-image of the center part showing a 320 nm wide, 100 nm high AFM-written wire connecting the 2 µm wide laser-written leads.
175
0 nm
17
5
m 0n
Height (nm)
12 8 4 0
0
100
200
300
400 nm
500
Fig. 2. A grid pattern fabricated with the AFM. After etching the lines are 50 nm wide and 10 nm high.
600
700
558
Superlattices and Microstructures, Vol. 20, No. 4, 1996 B
Normal scanning gh
t
A
5 4
*
* *
3
* *
2 1 0
ght
Applying a voltage
Change in force (nN)
La se
r li
6
La se
r li
–10
*
*
* *
–4 –2 –8 –6 Applied voltage (V)
0
– – – – – – + + + + + +
Fig. 3. A, A schematic of the bending of the cantilever without and with an applied voltage. B, The graph shows the dependence of tip displacement on applied voltage. The solid curve is a fit using the expression in the text.
a voltage is applied to the tip while scanning, electrostatic forces attract the tip and cantilever towards the scanned surface. This attraction causes a bending of the cantilever, resulting in a change of the deflected laser beam in such a way that the feedback system of the AFM responds as if an additional repulsive force is present. In Fig. 3A a sketch shows the bending of the cantilever with and without applying a voltage. The graph in Fig. 3B shows the changes in repulsive force as a function of applied voltage. The changes in repulsive force are calculated from the measured difference in the deflection of the cantilever with and without an applied voltage divided by the force constant of the cantilever. The points are an average of ten measurements. There is no difference whether a negative or positive voltage is applied, which agrees with the notion that the electrostatic forces are always attractive. Further, Fig. 3B shows a nonlinear dependence of the applied voltage which can be modeled simply in terms of a capacitor with area A, plate separation d, applied voltage V , and constant of vacuum permittivity ε such that the electrostatic force between the plates is Fe =
ε AV 2 . 2d 2
The continuous line in the figure is for d = 15 nm and A = 542 nm2 in reasonable agreement with the dimensions of the tip apex. To model this more accurately requires a full calculation of the electrostatic forces for the complex geometry of tip, cantilever and surface. The simple formula quoted in the text suggests that the force is produced mainly between tip apex and surface. The result of the electrostatic forces are a considerable wear of the coated tip. We are investigating ways to compensate for the electrostatic attraction, to increase the lifetime of the tips.
4. SNOM lithography Based on the results obtained with the laser direct writing, we have also used a scanning near-field optical microscope (SNOM) for surface illumination [16]. By operating in the optical near-field the SNOM overcomes
Superlattices and Microstructures, Vol. 20, No. 4, 1996
10.4 nm
11.3 nm 3500 nm
1050 nm
B
3500 nm
A
559
1050 nm
Fig. 4. AFM images after etching of lines written using the SNOM. A, Lines written with light coupled into the uncoated probe. B, 50 nm wide lines resulting from scanning without light coupled into the fiber.
the diffraction limit of conventional optical microscopy. Figure 4A shows a part of a grating consisting of 32 parallel lines after etching in KOH. The lines are written using the SNOM with a scan speed of 80 µm s−1 on hydrogen-passivated amorphous silicon. The surface is locally illuminated through an uncoated tapered fiber probe using an argon ion laser running at a wavelength of 458 nm. The far-field power emitted from the SNOM-probe is measured to be 25 mW. This again leads to a local oxidation of the silicon. Figure 4A shows a part of the total exposed area. Each scan line is composed of three lines, a center line of 110 nm and two broader lines of 240 nm on each side. The narrow line is believed to originate from the near-field light exposure, whereas the broader side lines result from interference with the optical far-field which penetrates the sidewalls of the uncoated probe [16]. Unexpectedly, linewidths of 50 nm have been obtained without light coupled into the probe. The hydrogen desorption may in this case be caused by an electrostatic potential between the probe and the amorphous silicon layer which is electrically isolated from the Si(100) substrate. Thus the SNOM appears to combine both methods of local oxidation, light-induced and field-induced, which are achieved separately by laser direct writing and AFM. One of the advantages of using the SNOM for lithography is that the wear of the tip is avoided, since the probe-to-sample distance is controlled by a shear-force microscope [17].
5. Conclusion In conclusion we have presented a novel combination of direct laser and AFM lithography for connecting nanometre size structures with micrometre and millimetre-scale circuitry. The technique offers a simple approach to patterning amorphous silicon surfaces by local oxidation of silicon films and subsequent etching.
References [1] L. A. Nagahara, P. I. Oden, A. Majumdar, J. P. Carrejo, J. Graham and K. Alexander, SPIE Scanning Probe Microscopies 1639, 171 (1992). [2] L. L. Sohn and R. L. Willet, NT2 FRIDAY, p. 1–4. [3] M. Wendel, S. K¨uhn, H. Lorenz and J. P. Kotthaus, Appl. Phys. Lett. 65, 1775 (1994).
560 [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Superlattices and Microstructures, Vol. 20, No. 4, 1996 E. S. Snow and P. M. Campbell, Appl. Phys. Lett. 64, 1932 (1994). D. Wang, L. Tsau and K. L. Wang, Appl. Phys. Lett. 67, 1295 (1995). S. C. Minne, Ph. Flueckiger, H. T. Soh and C. F. Quate, J. Vac. Sci. Technol. B 13, 1380 (1995). E. S. Snow, W. H. Juan, S. W. Pang and P. M. Campbell, Appl. Phys. Lett. 66, 1727 (1995). G. C. Abeln, T. C. Shen, J. R. Tucker and J. W. Lyding, Microelectronic Engineering 27, 23 (1995). K. Matsumoto, M. Ishii, K. Segawa and Y. Oka, Appl. Phys. Lett. 65, 34 (1996). S. C. Minne, H. T. Soh, Ph. Flueckiger and C. F. Quate, Appl. Phys. Lett. 66, 703 (1995). M. M¨ullenborn, H. Dirac J. W. Petersen, Appl. Phys. Lett. 66, 3001 (1995). N. Kramer, M. Niesten and C. Sch¨onenberger, Appl. Phys. Lett. 67, 2989 (1995). M. M¨ullenborn, K. Birkelund, S. Madsen and F. Grey, Appl. Phys. Lett., to be published. L. Tsau, D. Wang and K. L. Wang, Appl. Phys. Lett. 64, 2133 (1994). H. C. Day and D. R. Allee, Appl. Phys. Lett. 62, 2691 (1993). S. Madsen, M. M¨ullenborn, K. Birkelund and F. Grey, Appl. Phys. Lett., to be published. E. Betzig, P. L. Finn and I. S. Weiner, Appl. Phys. Lett. 60, 2484 (1992).