SPM-Based Nanotechnology
Two decades ago the family of surface modification and patterning methods was enhanced with SPM-based nanotechnology, mainly represented by STM and AFM nanolithography [1456, 1457]. Since that time, proximal probe nanolithography increasingly often appears in the focus of interest of semiconductor device developers and draws the attention of the rest of the scientific community. This particular interest is due to the fact that nowadays state-of-the-art SPM equipment allows for direct scripting on surfaces in very accurate and well-controllable manner. Thus, it tends to become a promising and competitive alternative or useful addition to the well-established methods of electron-beam lithography and wet chemical etching.
The first experiments on surface modification by means of proximal-probe techniques are performed by Dagata et al. [1027, 1312]. The technique of local anodic oxidation (LAO), proposed in these works, now is an approved one for writing very small oxide patterns on both metallic and semiconductor surfaces. Research groups worldwide employ this technique for the fabrication of nanoelectronic devices, nanoelectromechanical systems, electro-optical structures, and templates for chemical and biological self-assembly. According to National Institute of Standards and Technology, this method is given its own unique code, 81.16.Pr, in the regularly updating American Institute of Physics PACS (Physics and Astronomy Classification Scheme) database.
The phenomenon of anodic nano-oxidation has been proven to be of electrochemical nature. The simplified mechanism is as follows. The thin water film covering the surface under monitored and controlled ambient humidity of about 50% serves as an electrolyte between the sample (anode) and the tip (cathode). The negatively biased tip (though in the first STM-based work [1027] positive biasing is used) imposes an external electric field which dissociates water molecules into oxidizing species, presumably OH–. The field further enhances the vertical drift of these species away from the tip towards the surface where they react with the underlying atoms to form a localized oxide beneath the tip. The field strength decays across the growing oxide film and the oxide growth process self terminates at/below the critical electric field of the order of 109 V/m [159]. The growth rate of oxide occurs to be strongly dependent on the bias applied to the tip [182]. The growing oxide expands in both up and down directions relative to surface of the sample, the depth being slightly more than the protrusion.
Nanotechnology of Silicon
A detailed mechanism of local oxidation particularly for silicon is investigated in the papers of Gordon et al. [1467], Avouris et al. [1078, 1468], Dagata et al. [1474].
The "dry" technique of local oxidation involves removing hydrogen atoms on an H-passivated silicon surface by means of an STM or AFM tip in vacuum. Depassivated areas can then be oxidized when exposed to an air environment due to the high reactivity of bare silicon [1456].
Using contact mode AFM Lee et al. [1462] mechanically remove the passivating hydrogen layer by means of a silicon nitride AFM cantilever applying forces of about several tens nN for subsequent oxidizing with ambient oxygen.
Campbell et al. [182] suggest that using AFM with a conductive tip offers the advantage over STM in that the exposure and imaging mechanism are decoupled. This guarantees independent writing and subsequent imaging of the written pattern without fear of additional exposure.
The ambient relative humidity (RH) affects drastically the aspect ratio of written patterns due to varying of the water film thickness on the surface of the specimen as shown in a paper of Avouris et al. [1468] who perform nanooxidation experiments within the range of 10-95% RH. It is revealed that the water film not only supplies the oxidizing species, but its finite conductance also leads to a defocusing of the electric field, which degrades the lateral resolution of the process. The highest aspect ratio structures form at about 14% RH though an optimal value from a different source is considered to be about 50% RH.
By the opinion of Moon et al. [1489] control of the hydrophilicity would enlarge the applicability of LAO technique to practical applications where fabrication and patterning of thicker oxide are required. A significant difference is found for nanooxidation of silicon surfaces treated beforehand with hydrophilic (NH4OH:H2O2:H2O=1:1:10 known as SC1) and hydrophobic (HF:H2O=1:100) solutions which apparently affect the parameters of the water layer on the oxidizing surface. It is found that the average height (width) of the oxide line is 4.8nm (550nm) for the former surface and 1.9nm (340nm) for the latter one for a bias voltage of 18V and a scan rate of 2.3µm/s. Thus, there is another way to control water layer parameters and, therefore, growing oxides besides ambient humidity.
Nanoscale pits in silicon fabricated by anodic oxidation are considered as a possible nucleation sites in crystal growth and regions for metal embedding for quantum devices [1490].
Besides SiO2, there is another widely used electronic material possessing higher dielectric constant. It is Si3N4 or SixNy due to its usual nonstoichiometry. Having in mind its usefulness as a material for nanolithography Pyle et al. [1469] and Workman et al. [227] study the growth of silicon nitride lines on the H-terminated silicon substrate in the atmosphere of NH3. It is shown that Faraday current also exist during growth pointing to the electrochemical nature of this process. That gives rise to deduction of a possible growth mechanism of nanolithographed nitride lines. Following this model, numerical evaluations reveal low efficiency of the process attributed to the presence of a tunneling current that could be diminished by an additional insulating layer on the tip apex. The authors believe that the value of the Faraday current probably can be used to control the growth rate.
Nanotechnology of silicon, thus, continues attracting much attention and is worthy of separate brief description. By itself it can hardly be considered as sufficient for fabrication of a complete device. It should be rather recognized as an intermediate stage for the fabrication of more complicated nanostructures involving metal patterning, various etching techniques, etc. Possible applications of these oxide patterns are quantum and molecular devices, optical gratings, sites for immobilization of biological macromolecules and a many others.
Nanotechnology of AIIIBV semiconductors
Multilayer semiconductor heterostructures based on AIIIBV compounds represent a relatively novel class of objects in which the electrons are confined to a very thin, high mobility area, the so-called 2-dimensional electron gas (2DEG), underneath the surface, the depth being in order of several tens nanometers. Using these structures one can build devices, which rely on the discreteness of the electronic charge.
For instance, single-electron transistors (SETs) defined in a two-dimensional electron gas of a semiconductor heterostructure known also as quantum dots (QDs) are of growing interest. Compared to the various other methods of fabrication of such devices AFM nanolithography provides many advantages, in particular small lateral depletion lengths and high specularity of the scattering at the boundaries [1472].
A double-dot SET fabricated by Lüscher et al. [130] exhibits steep potential walls and a lateral depletion length of less than 20nm, compared with the 100nm value for an electron-beam lithography defined device. It is also shown, that operating the QDs with positive top gate voltages results in excellent transfer of the lithographic pattern into the electron gas.
Oxide lines written on the surface of multilayer AIIIBV devices with delta-doped structure can serve as electrical barriers in the patterning of electronic devices. The main advantage of this technique over the more conventional and well-established split-gate technique is the strong lateral confinement provided by oxide lines, resulting in relatively high sub-band energy spacing. This way it is possible even to constrict a 2DEG at geterojunction interface into a narrow one-dimensional electron channel which exhibits quantized conductance at low temperatures as reported by Nemutudi et al. [159].
Other experiments on modification of 2DEG are reported by Sasa et al. [191, 219] in application to GaSb/AlGaSb/InAs heterostructures. After nanooxidation samples are immersed in water in order to remove oxides. Authors showed feasibility of formation of periodical structures modulating 2DEG and noted that etching speed of oxidized GaSb and AlGaSb layers increases greatly if compared with non-oxidized ones.
Fabrication of oxide wires on heavily doped p-type GaAs and n-type InGaP is investigated by Matsuzaki et. al [115]. A threshold bias voltage needed to produce the wires varies from 3 to 35 volts depending on the tip size. Dimensions of the wires are below 30nm. Later, 10nm size is reached using a special a-C tip formed on the apex of usual cantilever by electron beam deposition [107].
Held et al. [1475] suggest that local anodic oxidation of high electron mobility transistors provides a novel method to define nanostructures and in-plane gates. Two examples, namely antidots and quantum point contacts as in-plane gate transistors are fabricated and their performance at liquid nitrogen temperatures is discussed.
Nanotechnology of metals
Beginning from the earliest works such as that of Sugimura et al. [1476], SPM-based nanotechnology is successfully applied for modifying metal surfaces as well: Ti [159, 181, 1037, 1459, 1461, 1493], Nb [1461, 1473, 1494, 1495, 1496, 1497], V [631], Cr [1492], Ni [1195], Al [1460, 1498] and Au [880, 1491].
Vaccaro et al. [631] state that vanadium is a probable alternative to commonly used titanium since the oxidized line width is smaller and the height is higher for the former under the same conditions.
Boisen et al. [1460] use aluminium and aluminium oxide written by AFM nanolithography as reactive ion etch (RIE) masks for simple fabrication of suspended submicron silicon and silicon oxide structures. Aluminium is a commonly used metal in complementary metal-oxide-semiconductor (CMOS) device fabrication. Furthermore, RIE is a widely used etch technique for CMOS processing whereas KOH is not suitable due to the consequent contamination by potassium ions. Hence, this patterning technique is CMOS compatible, making it in principle, possible to fabricate advanced nanomechanical structures with integrated microelectronics. Two years earlier E. S. Snow et al. [1498] manufacture atomic point contacts by using anodic oxidation of thin Al films. Quantized decrease in conductance of Al nanowires during final stages of anodization in discrete steps of ~2e2/h is observed.
Several groups report fabrication non-AIIIBV quantum devices such as metal-oxide SETs mentioned above [1453, 1473, 1496, 1497] metal-insulator-metal junctions [181, 1495, 1493], tunneling barriers [1459, 1461].
Shmidt et al. [1461] investigate another route for oxidizing thin metal Ti and Nb films with nanometer-scale resolution. The process called Current-Induced Local Oxidation (CILO) involves applying high in-plane current densities to very narrow metal strips (in order of 100 nm wide) embraced by oxide regions previously fabricated by means of standard AFM nanooxidation, which results in the formation of an insulating oxide barrier between these two regions in a self-limiting fashion. In this way a tunneling metal-oxide-metal junction appears. The authors note that the current-induced oxidation occurs under conditions entirely different from those in AFM tip-induced oxidation. In the latter case, the oxidation is forced by high electric fields of 107V/cm oriented perpendicularly to the surface, which drive the oxidant into the film. In contrast, the stress voltage during the CILO process leads to electric fields of 105V/cm, which are not only orders of magnitude lower but also oriented in the plane of the film. Towards the end of the barrier formation, only atomic-scale channels remain unoxidized. Despite a strongly increased current density, the oxidation rate decreases drastically and the conductance drops in steps of about 2e2/h. This gives evidence of ballistic transport and superior stability of such metallic nanowires against current-induced forces as compared with the bulk metal, which may be of great importance for future atomic-scale electronic devices.
For soft metals a direct method of mechanical machining can be used which is considered separately in the following paragraph in respect to nonmetal materials.
Modification of thin gold films can serve as an example. Schumacher et al. [880] explore contact AFM in constant force mode to create holes and grooves in thin Au films. When small forces in the order of nN are applied, displacement only of small grains takes place, and when applying µN forces it is possible to create deep groves down to the mica substrate. A set of cantilevers with spring constants between 0.05 and 25 N/m is used for these experiments.
Nanotechnology of soft materials
Modification of relatively soft materials can be performed using direct nanoscratching by means of an AFM probe tip as partially described above for semiconductor heterostructures and Au coatings. Another way is to use electron sensitive organic compounds that can be modified under appropriate voltages applied by a conductive AFM tip or STM tip.
Simple preparation of Langmuir-Blodgett (LB) films and Self-Assembled Monolayers (SAM) deposited on different substrates is the easiest way for nanoscale patterning that has many advantages over conventional electron beam lithography and photolithography. All above-mentioned materials can serve as substrates for subsequent development. Applicability of nanooxidation and local elecron-beam lithography to this systems depends upon a number of factors. For instance, the thickness of the covering organic layers must be small enough due to the limited penetration depth of low energy electrons emitted via STM or AFM probes. Their chemical composition should also be properly adjusted. For example, some terminal groups in organic SAMs may prevent oxidation and others may enhance it [259]; various metal cations may affect in a different way the anodization process for inorganic SAMs [1485]; various mixture ratios for preparation of LB films results in different anodization conditions [1471].
Takano and Fujihira [290] investigate lithography of thin chromium films on a quartz plate covered with a LB film (21 monolayers thick) of omega-tricosenoic acid as a resist. The authors use mechanical nanoscratching for writing patterns with subsequent reactive ion etching of cromium film through the grooves defined. Normal forces as small as 0.5nN is needed for removal of the resist.
For aurum nanopatterning Haesendonck et al. [1482] utilize the same compound as an elecron sensitive resist. LB film consisted of mearly 4 monolayers and was 12nm thick. It is known that omega-tricosenoic acid acts as a negative low-contrast resist when exposed with a high-energy electron beam. In this work exposure of such LB film is performed with the electron beam being emitted by an STM tip. After a detailed study of the influence of the exposure conditions on the quality of the fine-line patterns it is found that the optimum voltage for the exposure is close to 10V. Lower voltages often cause incomplete exposure of the resist layer whereas higher voltages result in an undesirable increase of the minimum achievable line width. The authors claim that for an average quality tip linewidths of less than 50nm could easily be achieved. In order to write the desired structures, a computer controlled pattern generator to obtain the x and y motion of the STM tip is designed. In contrast to the classical high-energy electron beam lithography, topographic images can also be obtained with the STM since the resist does not suffer exposure under voltages below 5V. A similar approach is proposed in the paper of Wilder et al. [1499] where 30nm wide features are formed in a 65nm thick resist by means of AFM conductive tip and then transferred through reactive ion etching into the silicon substrate.
Lee et al. [259] use AFM nanooxidation to pattern the silicon surface through SAMs of different composition prepared on a negatively charged silicon wafer. The monolayer thickness does not exceeded 21Å. It is found that results of nanopatterning under the same conditions depend upon the time of SAM formation and its chemical composition (namely, the nature of terminal groups). Complete coverage is achieved in adsorption time of 5 and 10min with ideal film thickness of 17-18Å. Oxide patterns written at relatively high rates of 0.36mm/s and applied voltage of 20V (corresponding current is 13nA) is observed by subsequent AFM as 118nm thick uniform lines 20 Åin height.
Systematic discussion on the peculiarities of nanometer scale mechanical patterning of amorphous polymer surfaces with AFM tips in ambient conditions and in liquid cells is presented in the work of Pickering and Vancso [660]. A number of references can be found in this paper on surface modification of various oligomeric and polymeric systems. The authors undertake a critical review of contemporary data available for glassy polymers revealing the most common achievements as well as problems in this field.
Concluding remarks
Common obstacles to be overcome in all nanolithography experiments as well as on the way to commercialization are wearing of SPM tip during the process and slow rates of writing.
H. Dai at al. [1619] believe that the remedy is in use of nanotube probes possessing extraordinary mechanical and electric characteristics. Extremely thin patterns of several nanometer wide can be written by such probes without noticeable wearing. As authors claim, slow rates of writing (0.1mm/s in 1996 [1463]) could be increased fivefold using nanotubes as compared with conventional tips and amount to 0.5mm/s. A year later Snow and Campbell [1470] declare successful experiments on oxidation and metal silicide processes at speeds approaching to 10cm/s (see also related paper [1454]).
All investigations from above cited references concerning oxidation of AIIIBV compounds were performed in tapping mode nanooxidation AFM. Self-limiting nature of this process does not allow for high scanning rates, which often does not exceed 100-200 nm/s. At the same time, there is a number of works devoted to manufacturing of heterostructure-based devices by contact mode AFM or nanoscratching that make higher scan rate possible. Schumacher et al. [192] report fabrication of SET by this technique applying contact forces within 50-100 mN at a scan velocity of 100µm/s, i.e. 2-3 orders of magnitude faster as compared with nanooxidation. The machining process is performed pressing the usual silicon non-contact AFM tip against the surface while multiply scanning along a line over the preformed Hall structure. After about 100 scan lines a local removal of the surface layers was achieved that is comparable to a shallow etch process.
Prospects in using SAMs and LB films for patterning are quite promising provided that the problems of layers uniformity, chemical stability and absence of nanoscopic defects such as pinholes are successfully solved. In contrast to LB films, SAMs require the proper functional groups for chemisorption to the substrate and proper chemical structure in order to be sufficiently affected under relatively weak electron beams emitted by STM or AFM tips.
Advances in nanooxidation allowed for demonstrating a possibility for fabrication of an extradense data storage media having fantastic capacity of 1.6Tbit per square inch [1464]. Bits pattern on atomically flat titanium surface was written in the form of an array of regularly arranged oxide cones of about 1 nm in height and 6-8 nm in width. To produce such a fine structure with the pitch between individual bits as small as 20 nm, single-walled nanotube tips with 2-5 nm diameters were used. Similar array was fabricated by conventional cantilever three years earlier [1463] with the following parameters: 30nm width, 1nm height, 100nm pitch. This outmost achievement though will hardly be developed even into prototype device due to a number of technical issues to be addressed but thereby was demonstrated a necessity to apply further efforts to approach the commercialization of nanolithography in less exotic applications.
An idea of parallelizm in respect to SPM-based nanotechnology is declared elsewhere (see, for example [1499]) and have to be implemented in viable commercial prototype as always has been demonstrated in application of AFM to data storage.
Successful fabrication of various nanodevices, steady improvement of accuracy and feasibility of SPM apparatus and scanning control techniques envisage further development of SPM-based nanotechnology.
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| ID | Reference list (newly come references are marked red) |
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| 219 | AFM fabrication and characterization of InAs/AlGaSb nanostructures S. Sasa, T. Ikeda, A. Kajiuchi, M. Inoue Solid-State Electronics, 42 (1998), 7-8, 1069-1073 |
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| 290 | AFM microlithography of a thin chromium film covered with a thin resist Langmuir-Blodgett (LB) film H. Takano, M. Fujihira Thin Solid Films, 273 (1996), 1-2, 312-316 |
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| 1196 | Fabrication of Semiconductor Nanostructures by Nanoindentation of Photoresist Layers Using Atomic Force Microscopy K. Wiesauer and G. Springholz J. Appl. Phys. 88 (2000), 7289-7297 |
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