SPM-based Nanotechnology

     A decade 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 more and more often appears in the focus of interest of semiconductor devices developers as well as draws attention of the rest scientific community. This particular interest is due to the fact that nowadays state-of-the-art SPM equipment allows for direct scripting on the surface in very accurate and well-controllable manner. Thus, it tends to become a promising and competitive alternative or useful addition to well-established methods of electron-beam lithography and wet chemical etching.

     First experiments on surface modification by means of proximal-probe techniques were performed by Dagata et al. [1027, 1312]. Technique of local anodic oxidation (LAO), proposed in these works, now is 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 recently was 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 proven to be of electrochemical nature. Simplified mechanism is as follows. The thinnest water film covering the surface under monitored and controlled ambient humidity of about 50% serves like an electrolyte between a sample (anode) and the tip (cathode). The negatively biased tip (though in the first STM-based work [1027] positive biasing was used) imposes an external electric field which dissociates water molecules into oxidizing species, presumably OH^–. The field further enhances vertical drift of these species away from the tip towards the surface where they react with 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 in strong dependence upon the bias applied to the tip [182]. Growing oxide is inflating in both up and down directions relative to surface of the sample, the depth being slightly more than the protrusion.

     Detailed mechanism of local oxidation particularly for silicon was investigated in papers of Gordon et al. [1467], Avouris et al. [1078, 1468], Dagata et al. [1474].

     "Dry" technique of local oxidation involves removing hydrogen atoms on H-passivated silicon surface by means of STM or AFM tip in vacuum. Depassivated areas can be then oxidized when exposed to an air environment due to high reactivity of bare silicon [1456].

     Using contact mode AFM Lee et al. [1462] mechanically removed the passivating hydrogen layer by means of silicon nitride cantilever applying the forces of about several tens nN for subsequent oxidizing with ambient oxygen.

     Campbell et al. [182] suppose that using AFM with a conductive tip offers the advantage over a STM that the exposure and imaging mechanism are decoupled. This guarantees independent writing and subsequent imaging of written pattern without fear of additional exposure.

     Ambient relative humidity (RH) affects drastically to the aspect ratio of written patterns due to varying of water film thickness on the surface of specimen as shown in paper of Avouris et al. [1468] who performed nanooxidation experiments within the range of 10-95% RH. It has been 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 the different point of view is considered to be about 50% RH.

     By the opinion of Moon et al. [1489] control of hydrophilicity would enlarge the applicability of LAO technique to practical applications where fabrication and patterning of thicker oxide are required. A great difference was 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 was found that average height (width) of the oxide line was 4,8 nm (550 nm) for the former surface and 1,9 nm (340 nm) for the latter one for a bias voltage of 18 V and a scan rate of 2,3 µm/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 Si_xN_y due to its usual nonstoichiometry. Having in mind its utility as a material for nanolithography Pyle et al. [1469] and Workman et al. [227] studied growth of silicon nitride lines on the H-terminated silicon substrate in the atmosphere of NH₃. It was shown that Faraday current pointing to the electrochemical nature of this process is also exists during the growth. That gave rise to deduction of a possible growth mechanism of nanolithographed nitride lines. If follow this model, numerical evaluations reveal low efficiency of the process attributed to the presence of a tunneling current that could be diminished by additional insulating layer on the tip apex. The authors believe that the value of Faraday current probably can be used to control the growth rate.

     Nanotechnology of silicon, thus, continues attracting much attention and was worthy of separate brief description. But itself it can hardly be considered as sufficient for fabrication of a complete device. It should be rather recognized as an intermediate stage for fabrication of more complicated nanostructures involving metal patterning, various etching techniques, etc. Possible applications of this oxide patterns are quantum and molecular devices, optical gratings, sites for immobilization of biological macromolecules and a many others.

     Multilayer semiconductor heterostructures based on A^IIIB^V compounds represent a relatively novel class of objects in which the electrons are confined to a very thin, high mobility area, 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. Among various methods of fabrication of such devices AFM nanolithography provides much more perfect characteristics, in particular small lateral depletion lengths and high specularity of the scattering at the boundaries [1472].

     Double-dot SET fabricated by Lüscher et al. [130] exhibited steep potential walls and a lateral depletion length of less than 20 nm if compared with 100 nm value for electron-beam lithography defined device. It has been shown also, 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 A^IIIB^V 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 one can easily possible even to constrict 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].

     Another experiments on modification of 2DEG were reported by Sasa et al. [191, 219] in application to GaSb/AlGaSb/InAs heterostructures. After nanooxidation samples were 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 were 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 were below 30 nm. Later, 10 nm size was reached using 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 have been fabricated and their performance at liquid nitrogen temperatures is discussed.

     Beginning from earliest works such 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], Au [880, 1491].

     Vaccaro et al. [631] state that vanadium is 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] used 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) electronics fabrication. Furthermore, RIE is a widely used etch technique for CMOS processing whereas KOH is inappropriate 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] made 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 was observed.

     Several groups reported fabrication non-A^IIIB^V 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] investigated 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 the very narrow metal strip (in order of 100 nm wide) embraced by oxide regions previously fabricated by means of "usual" AFM nanooxidation, which result in formation of insulating oxide barrier between these two regions in a self-limiting fashion. Thus tunneling metal-oxide-metal junction appears this way. Authors note that the current-induced oxidation occurs under conditions entirely different from the AFM tip-induced oxidation. In the latter case, the oxidation is forced by high electric fields of 10⁷ V/cm oriented perpendicular to the surface, which drive the oxidant into the film. In contrast, the stress voltage during CILO process leads to electric fields of 10⁵ V/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 a 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 direct method of mechanical machining can be used which is particularly considered in the following paragraph in respect to nonmetal materials.

     Modification of thin gold films can serve as an example. Schumacher et al. [880] explored contact AFM in constant force mode to create holes and grooves in thin Au films. When small forces in order of nN was applied, only small grains displacement took place, and while applying µN forces it is possible to draw a deep groves down to mica substrate. A set of cantilevers were used for this experiments with spring constants between k=0,05 N/m and k=25 N/m.

     Modification of rather soft materials can be performed using direct nanoscratching technique by means of a cantilever tip as was 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 conducting AFM or STM tip.

     Rather simple preparation of Langmuir-Blodgett (LB) films and Self-Assembled Monolayers (SAM) deposited on different substrates is the easiest way for nanoscale patterning that takes 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, thickness of the covering organic layers must be small enough due to limited penetration depth of low energy electrons emitted via STM or AFM probes. Their chemical composition also should be properly adjusted. For example, some terminal groups in organic SAMs may prevent oxidation and others enhance it [259]; various metal cations may affect different way on 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] investigated lithography of thin chromium films on a quartz plate covered with LB film (21 monolayers thick) of omega-tricosenoic acid as a resist. The authors used mechanical nanoscratching technique for writing needed patterns with subsequent reactive ion etching of cromium film through the grooves defined. Normal forces as small as 0,5 nN was needed for removal of the resist.

     For aurum nanopatterning Haesendonck et al. [1482] utilized the same compound as an elecron sensitive resist. LB film consisted of mearly 4 monolayers and was 12 nm thick. It is known that omega-tricosenoic acid act as a negative low-contrast resist when exposed with a high-energy electron beam. In this work exposure of such LB film was 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 has been found that the optimum voltage for the exposure is close to 10 V. Lower voltages often cause an incomplete exposure of the resist layer whereas higher voltages give rise to an undesirable increase of the minimum achievable line width. Authors claim that for an average tip quality the linewidths of less than 50 nm could always 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 has been designed. In contrast to the classical high-energy electron beam lithography, one can also obtain topographic images with the STM since resist does not suffer exposure under voltages below 5 V. A similar approach was declared in the paper of Wilder et al. [1499] where 30 nm wide features was formed in 65-nm-thick resist by means of AFM conducting tip and then transferred through reactive ion etching into the silicon substrate.

     Lee et al. [259] used AFM nanooxidation technique to pattern silicon surface through SAMs of different composition prepared on the negatively charged Si-wafer. Monolayer thickness does not exceeded 21Å. It was 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). The complete coverage was achieved in adsorption time of 5 and 10 min with ideal film thickness of 17-18Å. Oxide patterns written at relatively high rates of 0,36 mm/s and applied voltage of 20 V (corresponding current was 13 nA) was observed by subsequent AFM as 118 nm thick uniform lines of 20Å in height.

     Systematic discussion on the peculiarities of nanometer scale mechanical patterning of amorphous polymer surfaces with AFM tips in ambient conditions and in liquid cell is presented in the work of Pickering and Vancso [660]. A number of references could be found in this paper on surface modification of various oligomeric and polymeric systems. The authors undertook a critical review of contemporary data available for glassy polymers revealing the most common regularities as well as problems in this field.

     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 claime, slow rates of writing (0,1 mm/s in 1996 [1463]) could be increased fivefold using nanotubes as compared with conventional tips and amount to 0,5 mm/s. A year later Snow and Campbell [1470] declare successful experiments on oxidation and metal silicide processes at speeds approaching to 10 cm/s (see also related paper [1454]).

     All investigations from above cited references concerning oxidation of A^IIIB^V 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 100 µm/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 1,6 Tbit 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: width - 30 nm, height - 1 nm, pitch - 100 nm. 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.