Nanolithography on SiO2/Si with a scanning tunnelling microscope

Download PDF
4 downloads 0 Views 2MB Size Report
Comment
a thermal desorption procedure has been reported [19]. The advantages of using the high-energy electron beam technique are that it may utilize the techniques ...
INSTITUTE OF PHYSICS PUBLISHING

NANOTECHNOLOGY

Nanotechnology 14 (2003) R55–R62

PII: S0957-4484(03)35128-1

TOPICAL REVIEW

Nanolithography on SiO2/Si with a scanning tunnelling microscope Hiroshi Iwasaki, Tatsuo Yoshinobu and Koichi Sudoh The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki 567-0047, Japan E-mail: [email protected]

Received 6 May 2003, in final form 24 July 2003 Published 23 September 2003 Online at stacks.iop.org/Nano/14/R55 Abstract This paper gives a brief review of our recent work on a new method of nanoscale pattern formation of thin oxide film on Si substrate by using a scanning tunnelling microscope (STM): a field-emitted electron beam (e-beam) extracted from the STM tip is used for selective removal of the oxide film by e-beam-induced reduction and thermal annealing at moderately high temperatures (300–700 ◦ C). The process is dependent on electron dose and the patterning is controllable by adjusting the emission current and exposure time. One can draw nanoscale open-window patterns directly on oxide-covered Si substrates, e.g. lines and concentric circles of a few tens of nanometres in line width and spacing. Such patterning on the Si oxide layer shows good reproducibility and flexibility of the nanofabrication method, which suggests a further development and application of this method in nanotechnology. The beam profile of the extracted e-beam is measured and the beam-energy dependence of the quantum yield of the process is derived. Based on the excitation function, we consider that the decomposition is activated by core-level excitations as is the Knotek–Feibelman mechanism. One can also make use of this technique to diagnose the Si/SiO2 interface topography on a sub-nanometre scale. (Some figures in this article are in colour only in the electronic version)

1. Introduction The scanning tunnelling microscope (STM) has been developed not only as a useful tool for studies of surface morphology/structures and surface electronic structures, but also for new techniques for fabrication of nanostructures on surfaces. The STM nanofabrication techniques have been addressed in a number of reviews [1–5]. They range from the manipulation of single atoms [1, 2] to mesoscopic scale lithography in ultra-high vacuum (UHV), air and even liquid [3–5]. The physical principles of STM nanofabrication techniques include a wide range of phenomena such as mechanical contact, field evaporation, electron-beam(e-beam-) induced effects and anodic oxidation. To name just a few examples, Xe adatoms were transferred from a Pt(111) or a 0957-4484/03/110055+08$30.00 © 2003 IOP Publishing Ltd

Ni(111) surface to a tip by the transfer-on-contact process [6]. Mamin et al [7] formed ordered arrays of mounds on an Au surface in an STM with an Au probe tip by applying electric field above a threshold. Lyo and Avouris [8] demonstrated that Si atoms could be reversibly transferred between a Si surface and a W tip by field evaporation. STM has also been used for e-beam-induced lithography down to the nanoscale regime: this has included resist patterning [3], selective desorption of hydrogen from an H-terminated Si surface [9] and selective chemical vapour deposition [10], among many other examples. The group of Dagata [11] introduced a new method to write oxide line patterns on the surface of semiconductors by anodic oxidation in air using an atomic force microscope as well as an STM. Sugimura et al [12] expanded the method, which utilized a Ti surface that could be selectively oxidized.

Printed in the UK

R55

Topical Review

In this review article, we describe a new method of pattern formation on a SiO2 overlayer on Si using STM through irradiating the oxide overlayer by field-emitted e-beam from the STM tip at a moderate temperature in a UHV [13–18]. As Si oxide is an important electronic material, nanofabrication of this material without employing complicated traditional e-beam resist procedures can be an important issue for the development of nanoscale or single-electron devices for the next-generation electronics. Processing in UHV would be beneficial for nanofabrication on an atomically clean Si surface such as selective formation of a two-dimensional selforganized structure and hetero-epitaxial film overgrowth on it. Such an in-UHV processing cannot be carried out by ordinary e-beam lithography as it involves resist patterning and etching in chemicals. Recently, oxide removal with a focused electron beam of much higher energies (30 keV) followed by a thermal desorption procedure has been reported [19]. The advantages of using the high-energy electron beam technique are that it may utilize the techniques that are well developed in electron microscopy and electron beam lithography, and that it may achieve a nanometre scale focus of the e-beam. The present STM e-beam lithography technique has, however, some advantages compared to the high-energy electron beam technique; one can examine the patterned structure in atomic detail by using the same STM tip as used for the patterning in situ, defect formation by e-beam irradiation may be less severe at much lower electron energies and the fabrication system is fairly simple as well as the cost being quite low. We demonstrate that nanometre-scale windows of 50 nm on average can be cut through Si oxide overlayers with a minimum size of ∼25 nm. With this method, line- and ringshaped window patterns are successfully formed on the Si oxide layer [15–18]. This presents a new way to perform surface fabrications with a combination of the scanning probe and electron beam techniques, which can be well controlled by selecting the beam energy and the irradiation flux. Furthermore, we show that this method is based on both a low-energy-electron-stimulated reaction and thermal desorption: STM–LEESR/TD. We evaluate the quantum yield of SiO2 decomposition caused by an e-beam from the apex of an STM tip over an electron energy range of 10–180 eV and found decomposition onsets at 40 and 120 eV [20, 21]. These onsets are close to those found previously for e-beaminduced SiO2 dissociation by Auger electron spectroscopy and electron-stimulated desorption. Based on the excitation function, we consider that the decomposition is activated by core-level excitations like the Knotek–Feibelman mechanism. As an application of this method, we study the interface roughness of SiO2 /Si(001) for gate oxides of 8 and 15 nm in thickness together with samples treated chemically just before thermal oxidation [22–24]. By STM observation and scaling analysis we demonstrate that the interface roughnesses of thermal oxides/Si substrates are similar to each other and to that of the chemical oxide/Si substrate prior to thermal oxidation, which reveals that the interface roughnesses of the oxides are determined by the processes prior to the gate oxidations.

2. Experimental details Our experiments are carried out with a commercial UHV STM system (JSTM-4600X from JEOL), which is equipped R56

Figure 1. Schematic diagram of the experimental set-up of the silicon oxide removal by electron beam irradiation at an elevated temperature by using an STM.

with a direct current heating facility for sample cleaning and annealing, and supplemented with a gas inlet for oxygen dosing. The pressure of the UHV STM chamber is kept below 2.0 × 10−8 Pa. The sample is cut from an n-type Si(001) wafer with an electrical resistivity of ∼2.0  cm, and is cleaned by ultrasonic treatment before loading into the chamber. It is preannealed at 650 ◦ C for around 10 h in the UHV chamber to remove surface contaminations. The original native Si oxide overlayer (∼1 nm in thickness) on the sample surface is mainly used, whereas similar fabrication results are also obtained on thermal oxide layers. Samples with a thicker oxide layer were cut from a Si(001) wafer on which 2.7 nm thermal oxide (measured by cross-sectional TEM) is formed at 900 ◦ C using the rapid thermal oxidation (RTO) technique. The STM tip is prepared by electrochemical etching of a 0.3 mm W wire in a 0.5 N NaOH solution and cleaned by e-beam bombardment and field evaporation in the UHV chamber. The STM–LEESR/TD fabrication is performed by operating our STM in the field emission mode in which a lowenergy e-beam of 10–250 eV in energy and up to 1 µA in current is extracted from the STM tip. To achieve this, the feedback servo of the STM is set to active so that the tip-tosample distance can be auto-controlled to obtain the preset current under a given sample bias. By exposing this e-beam to the Si oxide overlayer, with the substrate being kept at a temperature of ∼300–700 ◦ C, reaction can be stimulated, and Si oxide within the e-beam exposing area can be selectively removed. The experimental set-up is depicted in figure 1. With this process, windows are cut through the oxide layer. To reveal the processing results, both topographic and current STM images are taken using the same STM as for the fabrication, with the sample bias of ∼4 V and tunnelling current of 0.3 nA in general.

3. Results and discussion 3.1. Pattern formation In figure 2, a typical result on the fabrication of Si oxide with this STM–LEESR/TD processing technique is illustrated. The fabrication is performed on a thin native oxide layer with an e-beam of 150 eV in energy and of 50 nA in beam current, and with an exposure time of 50 s at a substrate temperature of

Topical Review

Figure 2. STM current and topographic images on a typical fabrication result from a native Si oxide layer with the LEESR/STM processing.

Figure 3. STM current image on a set of line windows fabricated on the Si oxide layer by PC control of the STM tip position.

Figure 4. Nano-fabrication of concentric ring-window patterns by PC control of the STM tip position.

625 ◦ C. From the STM images, clear atomic steps on the (001)oriented Si substrate are observed within the e-beam-irradiated area that shows evidently the selective removal of Si oxide by the low-energy e-beam irradiation and thermal annealing. By scanning along a line over the Si oxide surface while dosing the electron beam, line windows can also be formed on the oxide layer. Figure 3 shows a typical result of such fabrication on formation of three line windows with this process. Here, the e-beam parameters are set to 80 eV in energy and 10 nA in current, and the sample is kept at 693 ◦ C during the processing and imaging. It can be seen from figure 3 that the line windows of Si oxide are straight and uniform and the line edges are also straight and abrupt. The RMS fluctuations of the line edges are estimated to be smaller than 3.5 nm. The widths of the three line windows are measured to be 56, 38, and 25 nm, corresponding to the different line doses of 3.3, 2.5, and 1.7 × 10−3 C cm−1 for the three lines from top to bottom, respectively. This result shows an exposure dependence of the fabrication dimensions, which reveals a good controllability of the LEESR/STM nanofabrication. It is obvious that most of the physical parameters involved in this fabrication technique, such as the beam energy, beam current, and exposure, are adjustable. Thus by optimizing these parameters we believe that it is possible to develop this LEESR/STM method into a well controlled technique for nanotechnology. With similar PC-controlled patterning, ring-shaped windows are also fabricated as shown in figure 4. The rings are formed by moving the STM tip in a circle during

exposure of the surface to the e-beam. The e-beam parameters used in the fabrication are the same as in the line-window formation, as shown above, while the line dose is 3.8, 4.3, and 4.8 × 10−3 C cm−1 for the three rings from the innermost to outermost ring (with radii of 100, 200, and 300 nm), respectively. It can be seen from figure 4 that the window edges are basically clear and distinct, and the ring widths are less than 50 nm. The origin of the window width dispersion in both the line-window and circular-window formations can be attributed to e-beam current fluctuation and poor uniformity of the oxide layer. This ring patterning result exhibits a flexible feature of the LEESR/STM nanofabrication. In addition to the LEESR/STM nanofabrication on thin native Si oxide, nanoscale line windows were also fabricated on a 2.7 nm thermal oxide layer. Figure 5 shows a current image of such a fabrication result in which three line windows are formed on the RTO oxide layer. We have not studied the maximum thickness of oxide that could be removed by the present method: however, we can say that we can remove a thermal oxide layer as thick as 3 nm. The e-beam parameters used to form the line windows are 120 eV energy and 50 nA current, and the exposure times are 10, 5, and 2.5 s for the three lines from top to bottom, respectively. It can be seen from the image that atomic steps also appear within the e-beam-irradiated areas, indicating formation of line windows by the LEESR/STM processing. The lines are straight, around 50 nm in width, and with a RMS fluctuation of line edges less than 6.5 nm for the top two line windows, that indicates R57

Topical Review

Figure 5. STM current image on formation of line windows on the 2.7 nm RTO oxide layer.

acceptable quality of the nanofabrication on the industrycompatible thermal oxide with the LEESR/STM technique. 3.2. Mechanism of the selective removal of the oxide overlayer by STM–LEESR/TD Two simple pictures of the mechanisms can be considered concerning this LEESR process: one is an e-beam-stimulated reaction followed by the thermal desorption of the decomposed species, as Fujita et al [19] proposed in explaining their highenergy e-beam etching result; the other is the heating effect by the current, as the current density we used in this direct fabrication can be as high as 103 A cm−2 . It is well known currently [25] that thermal decomposition of Si oxide may occur at elevated temperatures around 700 ◦ C in a reaction SiO2 + Si (substrate) → 2SiO (volatile); around 700 ◦ C (1) in which consumption of Si atoms is involved and voids are generally formed through the Si oxide layer into the Si substrate due to process (1). It can be imagined that if in the LEESR process the heating effect plays a key role, the etched windows will exhibit similar features to the thermal voids. In contrast, an electron-induced reaction [19], 2SiO2 → O2 + 2SiO (electron excitation)

(2)

basically does not require consumption of Si atoms from the Si substrate, and will leave fabricated window features different from the thermal voids. We compare STM images (not shown) of the Si oxide windows etched with the LEESR process to those of the thermal voids formed solely by annealing of the Si oxide layer in [18]. By drawing the surface corrugation plots across the etched areas and the thermal voids, we find clearly that the etched windows are much shallower than the thermal voids: the depths of the thermal voids are of the order of 2 nm, while those of the etched windows are generally less than 1 nm. Therefore it is clear that etching with the LEESR process basically does not involve consumption of substrate Si atoms, in contrast to the expectation based on the assumption of a heating effect. In addition to the above comparison, it can also be imagined that the etching of a Si oxide layer with the LEESR process will be very sensitive to the intensity or density of the beam current, if the current heating mechanism plays a key role. The local R58

temperature around the e-beam-irradiated area can be directly related to the current intensity or density, which in turn will lead to different processing results when set differently. In fact, there are slight differences in the sizes of the patterns formed with different current densities but with similar electron dose [17]. This point will be also examined in detail in the next section. In our experiment, the surface around the processed area is scanned for several frames after each processing. From this sequential imaging, we observed that on a SiO2 /Si surface irradiated with an electron beam current, etching of the SiO2 layer often takes some time (2–10 min, or even longer) to complete [16]. This implies that the LEESR processing involves a series of processes, rather than a direct electrical field effect or heating effect. We also observe that there exists a threshold of e-beam dose (total dose, ∼current × time) in the LEESR processing. Below this threshold, the Si oxide overlayer cannot be removed even with a long annealing process after e-beam exposure, though current fluctuation change can be observed around such exposed areas. We confirm that the native oxide we used cannot be decomposed by heating alone at 630 ◦ C for as long as 24 h. Heating is, however, indispensable for the removal of Si oxide. We irradiate a Si oxide layer with an e-beam of 120 eV energy and 30 nA current for as long as 30 min while the sample is kept at room temperature. There is no formation of open windows and we cannot find any surface changes by STM observation. In the case where the sample is kept at 630 ◦ C, an exposure to an e-beam of 120 eV energy and 30 nA current leads to the formation of an open window within 1 s. These results support the view that the process is a low-energy-electroninduced surface reaction followed by thermal desorption of SiO. 3.3. Intensity profiles of a low-energy electron beam extracted from an STM tip by field emission and energy dependence of the quantum yield of oxide decomposition To understand the SiO2 decomposition/desorption mechanism is not only interesting from a physics point of view but also important to improve the controllability of the STM– LEESR/TD nanofabrication. In the following, we evaluate the quantum yield of SiO2 decomposition. The e-beam irradiation of the surface is carried out with a beam current of 10–50 nA and beam energy of 10–180 V. Firstly, the intensity profile of a low-energy e-beam extracted from an STM tip apex in the field emission mode is measured [26]. We assume that the decomposition of Si oxide with bombardment of low-energy electrons depends on electron dose but not current based on some observations described in section 3.2. This working hypothesis will be examined in the end of this section by comparing predictions with the experimental results. Then there exists a quantum yield for the decomposition of Si oxide with low-energy electrons, Y , which is constant within the e-beam-irradiated area at a fixed beam energy. Other simplifying assumptions are that the thickness, d, of the native oxide layer is uniform and can be considered as a constant value over the areas under study and the beam profile has a rotational symmetry when the e-beam is emitted from a cone-shaped metal tip. The size of

Topical Review

Figure 6. Experimental results of the e-beam profile (dot) and Gaussian curve fittings to the experimental data (solid line). A circular opening window is assumed to derive the average radius for each exposure time. The standard deviation of the Gaussian function σ is deduced directly from the fitting curve.

where i (r ) is the beam intensity at the opened oxide window edge r , e is the elementary electron charge, and NO is the density of oxygen in Si oxide. Accordingly, i (r ) = (NO · d · e/Y ) · 1/tr .

80

60 -7

50 40 30

Yield (x10-7 )

70

Yield (10 )

oxide area removed (radius of the opened circle, r ) by exposure to the low-energy e-beam is measured as a function of the e-beam exposure time, tr . Based on these assumptions, for the e-beam decomposition process, the following relation must conform: (3) (i (r ) · tr )/e · Y = NO · d

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 10 20 30 40 50 60 Electron energy (eV)

20

(4) 10

Thus by measuring the e-beam exposure time and the corresponding oxide window size, the plot 1/tr versus r can be drawn, which will well represent the field emission e-beam profile. Figure 6 gives two typical e-beam profiles (dots) obtained in our experiment. In both measurements, the energy of the low-energy e-beam is set to 100 eV, whereas the beam current I is 30 and 50 nA for the results in figures 6(a) and (b), respectively. By fitting the experimental results with the Gaussian distribution function i (r ) = i 0 exp(−r 2 /4σ 2 ), where σ is the standard deviation of the Gaussian function, it can be seen that the intensity profile of the low-energy e-beam emitted from an STM tip can be well described by the Gaussian distribution function. From the experimental results, the standard deviations are 14.4 and 8.7 nm for the two measurements, respectively. With the standard deviation, the peak intensity of the e-beam profile can be obtained with the relation i 0 = I/(4πσ 2 ), which gives 1.15 × 103 and 5.26 × 103 A cm−2 , for the two e-beam profiles shown in figure 6, respectively. The dependence of the low-energy e-beam profile on the total beam current can be explained as follows: when the bias voltage between the tip and sample is fixed while the beam current is changed from one value to another, the tipto-sample distance will be adjusted by the feedback control of the STM system to vary the electrical field at the tip apex to extract the desired current. Thus an e-beam profile of higher

0 0

50 100 150 Electron Energy (eV)

200

Figure 7. The quantum yield for impact-induced SiO2 decomposition as a function of the e-beam kinetic energy with beam currents of 10 nA ( ), 20 nA (♦), 30 nA ( ), 40 nA () and 50 nA (×). The solid curve represents the substantial yield.

◦

current density can be expected when a larger beam current is set, which demands higher electrical field between the tip and sample at a short tip-to-sample distance, as observed in our experiment. Our experimental results also show that the peak current density increases more steeply than proportionally to the emission current change. Figure 7 shows the quantum yield for impact-induced SiO2 decomposition as a function of the electron kinetic energy: the excitation function [20]. The beam current ranges from 10 to 50 nA, and it should be noticed that the values of the yield measured with various incident beam currents converge to almost a single value in the plot. This finding is in agreement with the report by Carriere and Lang [27]. They showed the curves of Si reduced Auger peak height as a function of the time of bombardment with various incident beam currents that can be reduced to almost a single curve by using the irradiation dose as the variable. R59

Topical Review

We also notice that the quantum yield has a measurable value at an incident electron energy of 20 eV, though in the case of 10 eV no open windows are formed. We consider that this corresponds to the fact that an e-beam of 20 eV energy could just produce O(2s) core holes by excitation of the O(2s) electron to an empty state close to the Fermi level. By increasing the electron energy, more final empty states would be available and more O(2s) core holes would be produced. From the quantum yield of oxide decomposition around 50 × 10−7 at an incident electron energy of around 150 eV, the SiO2 decomposition crosssection σ (Y /(number of SiO2 molecule/cm2 )) is estimated to be ∼10−21 cm2 . This value is comparable to the previously evaluated values by Auger experiment [27]. The quantum yield is also similar to that found for e-beam nanolithography of SiO2 at a high electron energy (30 keV) [28]. Thus, the absolute value of the quantum yield Y , as well as the excitation function, supports that the mechanism of the selective oxide removal in STM nanolithography is as mentioned above: (1) the oxide layer within the e-beam-exposed area is decomposed and reduced, and then (2) the reduced SiO is changed to volatile SiO and evaporated from the surface at elevated temperatures. These findings indicate that core-level excitations play an important role in the decomposition of the Si oxide layer by electron irradiation in the STM nanolithography process as they do in the Knotek–Feibelman mechanism. 3.4. Analysis of oxide/Si interface roughness utilizing LEESR/TD oxide removal By utilizing the STM–LEESR/TD method, we analyse SiO2 /Si(001) interfaces for oxides from semiconductorindustrial processes which include RCA-treated chemical oxides just before thermal oxidation and thermal gate oxides of 8 and 15 nm thicknesses [22–24]. We use n-type (001)-oriented Si wafers which were processed using a VLSI fabrication line. All the samples are first cleaned by the SC1 treatment (NH4 OH:H2 O2 :H2 O = 1:5:35 at 80 ◦ C for 15 min), HF treatment (HF:H2 O = 1:50 at 25 ◦ C for 1 min) and SC2 treatment (HCl:H2 O2 :H2 O = 1:1:7 at 52 ◦ C for 10 min) (RCA cleaned). A sample is picked up after the RCA cleaning. The other samples are subsequently oxidized in a furnace under the wet-oxidation condition either at 800 ◦ C to 8 nm thickness (8 nm-oxide) or at 850 ◦ C to 15 nm thickness (15 nm oxide). On the RCA-cleaned sample, a chemical oxide overlayer around 1 nm thick is present according to spectroscopic ellipsometry measurement. All the oxide films are thinned by controlled HF etching to a few angstroms to minimize the e-beam bombardment process just before the samples are transferred into the UHV chamber. Such a thin oxide film is effective for protecting the surface from contamination and oxidation in air for a short period [22]. The oxide films are then removed by field-emitted e-beam irradiation with raster scanning of the STM tip at a sample bias voltage of 100 V and a field emission current of 20–30 nA over a 260 nm × 260 nm square region while heating the sample at 300 ◦ C. The scanning time is 90 s, which corresponds to an R60

area dose of (3–5) × 103 C cm−2 . Subsequently, the exposed Si surface is observed with the same tip at 300 ◦ C. All the STM images in this work are taken at a sample bias voltage of +2 V and a tunnelling current of 0.3 nA. Figure 8(a) shows a typical STM topographic image of the oxide-removed regions of a 15 nm oxide sample which exhibits random bumps and hollows. From the enlarged image (figure 8(b)), one can see clearly the dimer rows which are characteristic of a clean Si(001) surface. The STM image is quite similar to those for the 8 nm oxide sample and also the RCA-cleaned sample. The interface may be regarded as consisting of terraces of several nanometres in size separated by atomic steps of 0.136 nm height. On the terraces, there are small islands and vacancies of a few nanometres in size. For comparison, we show in figure 9 SiO2 /Si(001) interfaces for the 15 nm oxide sample (a) and a chemical oxide film formed on a UHV-flash-cleaned Si surface by the SC1 treatment for 1 min (b) [22]. The topograph shown in figure 9(a) is much rougher than that shown in figure 9(b). We consider, together with the quantitative analysis of the interface roughness of the samples by scaling analysis, that the interface roughnesses of the oxides were determined by the processes prior to the gate oxidations.

4. Future work As described above, the STM nanofabrication technique on SiO2 is a promising technique as it does not require complicated traditional e-beam resist procedures. Furthermore, the STM technique allows one to observe the fabricated nanostructures with the same STM at the same location with atomic-scale resolution. The lower-energy e-beam employed by the STM technique may be beneficial to reduce possible e-beam bombardment damage sustained using a high-energy e-beam. To achieve the goal of developing a novel technique for nanolithography on Si oxide, based on this low-energy e-beam/STM method, we believe that substantial research should be conducted to achieve a systematic control and optimization of the physical parameters needed for improving the nanofabrication quality. As shown in the previous sections, most of the physical parameters involved in the STM–LEESR/TD method are controllable ones such as the beam energy, beam current, and exposure. Thus, to make this method be a well controlled technique for nanotechnology, we need a highly stable system. A thermal drift free STM system for various different operating temperatures is desirable for widening the applicability of the technique as well as for finer and more stable lithography. Development of reliable tips is very important for the improvement of the technique as the physical parameters are very sensitive to the tip-apex structures on an atomic scale. It would be interesting to use a carbon nanotube tip for this technique. Preparation of chemically and structurally homogeneous Si oxide films is also important to improve the reliability of the lithography. Developing a method to use other insulating films such as epitaxial thin CaF2 film may expand the applicability of this technique. As Si oxide is an important electronics material, nanoscale applications of this material have attracted great interest and

Topical Review

Figure 8. An STM topographic image (130 nm × 130 nm) of an oxide-removed region of the 15 nm oxide sample (a) and an enlarged image of part of it (b).

Figure 9. STM topographic images of the oxide-removed regions of (a) the 15 nm oxide sample and (b) a chemical oxide film formed on a UHV-cleaned Si surface by the SC1 treatment for 1 min.

led to various investigations [29, 30]. Using the nanofabricated SiO2 layer with a fine focused high-energy e-beam as a mask, subsequent MBE or chemical beam epitaxy (CBE) growths of nanostructures have been reported [31, 32]. Application of the STM–LEESR/TD method to diagnose SiO2 /Si interface roughness described in section 3.4 indicates that one can prepare atomically clean Si surfaces by heating the substrate at much lower temperatures than the usual temperatures around 1200 ◦ C. One may prepare clean Si surfaces in wide areas with the aid of an e-beam shower for Si substrates in which patterned delta-doping layers have been fabricated beforehand for nanodevice fabrication.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Acknowledgments The authors would like to thank Dr Nan Li for helpful discussions in preparing this article. This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (no 08247214) and the Centre of Excellence of Osaka University (no 09CE2005) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

[11] [12] [13] [14] [15]

Stroscio J A and Eigler D M 1991 Science 254 1319–26 Avouris P 1995 Accounts Chem. Res. 28 95–102 Pease R F W 1992 J. Vac. Sci. Technol. B 10 278–85 Quate C F 1997 Surf. Sci. 386 259–64 Baski A A 2002 Advanced Semiconductor and Organic Nano-Techniques part 3, ed H Morkoc (New York: Academic) Eigler D M, Lutz C P and Rudge W E 1991 Nature 352 600–3 Mamin H J, Guethner P H and Rugar D 1990 Phys. Rev. Lett. 65 2418 Lyo I-W and Avouris P 1991 Science 253 173 Lyding J W, Abeln G C, Shen T-C, Wang C and Tucker J R 1994 J. Vac. Sci. Technol. B 12 3735 Silver R M, Ehrichs E E and de Lozanne A L 1987 Appl. Phys. Lett. 51 247 Dagata J A, Schneir J, Harary H H, Evans C J, Postek M T and Bennett J 1990 Appl. Phys. Lett. 56 2001 Dagata J A 1995 Science 1270 1625 Sugimura H, Uchida T, Kitamura N and Masuhara H 1993 Appl. Phys. Lett. 63 1288 Li N, Yoshinobu T and Iwasaki H 1998 Japan. J. Appl. Phys. 37 L995 Shibata M, Nitta Y, Fujita K and Ichikawa M 1998 Appl. Phys. Lett. 73 2179 Li N, Yoshinobu T and Iwasaki H 1999 Japan. J. Appl. Phys. 38 L252

R61

Topical Review

[16] Li N, Yoshinobu T and Iwasaki H 1999 Appl. Surf. Sci. 141 305–12 [17] Li N, Yoshinobu T and Iwasaki H 1999 Appl. Phys. Lett. 74 1621 [18] Li N, Yoshinobu T and Iwasaki H 2000 Ultramicroscopy 82 97 [19] Fujita S, Maruno S, Watanabe H and Ichikawa M 1996 Appl. Phys. Lett. 69 638 [20] Ito T, Gotoh M, Sudoh K and Iwasaki H 2001 Japan. J. Appl. Phys. 40 6055 [21] Iwasaki H, Ito T, Gotoh M, Li N and Sudoh K 2001 Nanoscale Spectroscopy and its Applications to Semiconductor Research (Lecture Notes in Physics vol 588) p 231 [22] Gotoh M, Sudoh K and Iwasaki H 2001 J. Vac. Soc. Japan. 44 341 [23] Gotoh M, Sudoh K, Itoh H, Kawamoto K and Iwasaki H 2002 Appl. Phys. Lett. 81 430 [24] Gotoh M, Sudoh K and Iwasaki H 2002 Japan. J. Appl. Phys. 41 7293

R62

[25] E.g., see Liehr M, Lewis J E and Rubloff G W 1987 J. Vac. Sci. Technol. A 5 1559 Rubloff G W 1990 J. Vac. Sci. Technol. A 8 1853 [26] Kawamoto I, Li N, Yoshinobu T and Iwasaki H 1999 Japan. J. Appl. Phys. 38 6172 [27] Carriere B and Lang B 1977 Surf. Sci. 64 209 [28] Watanabe H, Fujita S, Maruno S, Fujita K and Ichikawa M 1997 Appl. Phys. Lett. 71 1038 Watanabe H, Fujita S, Maruno S, Fujita K and Ichikawa M 1997 Japan. J. Appl. Phys. 36 7777 [29] Allee D R, Umbach C P and Broers A N 1991 J. Vac. Sci. Technol. B 9 2838 [30] Wada T, Hirayama M, Haraichi S, Ishii K, Hiroshima H and Komuro M 1995 Japan. J. Appl. Phys. 34 6961 [31] Fujita K, Wananabe H and Ichikawa M 1997 Appl. Phys. Lett. 70 2807 [32] Shklyaev AA, Shibata M and Ichikawa M 1998 Appl. Phys. Lett. 72 320