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富国海底世界3.5 Nanotechnology and Micro-machine
3.5.1 Nanotechnology
l. Introduction
What is nanotechnology? It is a term that entered into the general vocabulary only in the late 70’s, mainly to describe the metrology associated with the development of x-ray, optical and other very preci components. We defined nanotechnology as the technology where dimensions and tolerances in the range 0.l— 100 nm (from the size of the atom to the wavelength of light) play a critical role.思念故乡的诗
This definition is too all-embracing to be of practical value becau it could include, for example, topics as diver as x-ray crystallography, atomic physics and indeed the whole of chemistry. So the field covered by nanotechnology is later narrowed down to manipulation and machining within the defined dimensional range (from 0.1 to 100nm) by technological means, as oppod to tho ud by the craftsman, and thus excludes, for example, traditional forms of glass polishing. The technology relating to fine powders also comes under the general heading of nanotechnology, but we exclude obrvational techniques such as microscopy and various forms of surface analysis.
Nanotechnology is an “enabling” technology, in that it provides the basis for other technological developments, and it is also a “horizontal” or “cross-ctional” technology in that one technique may, with slight variations be applicable in widely differing fields. A good example of this is thin-film technology, which is fundamental to electronics and optics. A wide range of materials are employed in devices such as computer and home entertainment peripherals, including magnetic disc reading heads, video castte recorder spindles, optical disc stampers and ink jet nozzles. Optical and miconductor components include lar gyroscope, mirrors, diffraction gratings, x-ray optics, quantum-well devices.
2. Materials Technology
The wide scope of nanotechnology is demonstrated in the materials field, where materials provide a means to an end and are not an end in them-lves. For example, in electronics inhomogeneities in materials on a very fine scale I t a limit to the nanometer-sized features that play an important part in miconductor technology, and in a very different field, the finer the grain size of an adhesive, the thinner will be the adhesive layer, and the higher will be the bond strength.
(1) Advantages of Ultra-fine Powders
In general, the mechanical, thermal, electrical and magnetic properties of ceramics, sintered别诗
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metals and composites are often enhanced by reducing the grain or fiber size in the starting materials. Other properties such as strength, the ductile-brittle transition, transparency, dielectric coefficient and permeability can be enhanced either by the direct influence of an ultra-fine microstructure or by the advantages gained by mixing and bonding ultra-fine powders.
Other important advantages of fine powders are that when they are ud in the manufacture of ceramics and sintered metals, their green (i. e. unfired) density can be greatly incread. As a conquence, both the defects in the final product and the shrinkage on firing are reduced, thus minimizing the need for subquent processing.
(2) Applications of Ultra-fine Powders
Important applications include:
1) Thin films and coatings – the smaller the particle size, the thinner the coating can be.
2) Chromatography – the increa in specific surface area associated with small particles, allows column lengths to be reduced.
3) Electronic ceramics – reduction in grain size results in reduced dielectric thickness.
4) Strength-bearing ceramics – strength increas with decreasing grain size.
5) Cutting tools smaller grain size results in a finer cutting edge, which can enhance the surface finish.
6) Impact resistance – finer microstructure increas the toughness of high-temperature steels.
7) Cements – finer grain size yields better homogeneity and density.
8) Gas nsors – finer grain size gives incread nsitivity.
9) Adhesives – finer grain size gives thinner adhesive layer and higher bond strength.
课堂观察记录3. Precision Machining and Materials Processing
A considerable overlap is emerging in the manufacturing methods employed in very different areas such as mechanical engineering, optics and electronics. Precision machining encompass not only the traditional techniques such as turning, grinding, lapping and polishing refined to the nanometre le
vel of precision, but also the application of ‘particle’ beams, ions, electrons and x-rays. Ion beams are capable of machining virtually any material and the most frequent applications of electrons and x-rays are found in the machining or modification of resist materials for lithographic purpos. The interaction of the beams with the resist material induces structural changes such as polymerization that alter the solubility of the irradiated areas.爱日
(l) Techniques
1) Diamond turning. The large optics diamond-turning machine at the Lawrence Livermore National Laboratory reprents a pinnacle of achievement in the field of ultra-precision machine tool engineering. This is a vertical-spindle machine with a face plate l.6m in diameter and a maximum tool height of 0.5m. Despite the large dimensions, machining accuracy for form is 27.5nm RMS and a surface roughness of 3nm is achievable, but is dependent both on the specimen
material and cutting tool.关于真诚的作文
2) Grinding. Fixed abrasive grinding. The term “fixed abrasive” denotes that a grinding wheel is employed in which the abrasive particles, such as diamond, cubic boron nitride or silicon carbide I are attached to the wheel by embedding them In a resin or a metal. The forces generated in grinding
are higher than in diamond turning and usually machine tools are tailored for one or the other process. Some Japane work is in the vanguard of precision grinding, and surface finishes of 2nm (peak-to-valley）have been obtained on single-crystal quartz samples using extremely stiff grinding machines.
Loo abrasive grinding. The most familiar loo abrasive grinding process are lapping and polishing where the workpiece, which is often a hard material such as glass, is rubbed against a softer material, the lap or polisher, with an abrasive slurry between the two surfaces. In many cas, the polishing process occurs as a result of the combined effects of mechanical and chemical interaction between the workpiece, slurry and polisher.
Loo abrasive grinding techniques can under appropriate conditions produce unrivalled accuracy both In form and surface finish when the workpiece is flat or spherical. Surface figures to a few nm and surface finishes to better than 0.5nm may be achieved. The abrasive is in a slurry and is directed locally towards the workpiece by the action of a non-contacting polyurethane ball spinning at high speed，and which replaces the cutting tool in the machine. This technique has been named “elastic emission machining” and has been ud to good effect in the manufacture of an x-ray mirror having a figure accuracy of 10 nm and a surface roughness of 0.5 nm RMS.
春节素材图片3) Thin-film production. The production of thin solid films，particularly for coating optical components, provides a good example of traditional nanotechnology. There is a long history of coating by chemical methods, electro-deposition, diode sputtering and vacuum evaporation, while triode and magnetron sputtering and ion-beam deposition are more recent in their wide application.
Becau of their importance in the production of miconductor devices, epitaxial growth techniques are worth a special mention. Epitaxy is the growth of a thin crystalline layer on a single-crystal substrate, where the atoms in the growing layer mimic the disposition of the atoms In the substrate。
The two main class of epitaxy that have been reviewed by Stringfellow (1982) are liquid-pha and vapour-pha epitaxy. The latter class includes molecular-beam epitaxy (MBE), which，in esnce，is highly controlled evaporation in ultra high vacuum. MBE may be ud to grow high quality layered structures of miconductors with mono-layer precision, and it is possible to exerci independent control over both the miconductor band gap, by controlling the composition, and also the doping level. Pattern growth is possible through masks and on areas defined by electron-beam writing.
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4. Applications
There is an all-pervading trend to higher precision and miniaturization, and to illustrate this a few applications will be briefly referred to in the fields of mechanical engineering, optics and electronics. It should be noted, how-ever, that the distinction between mechanical engineering and optics is becoming blurred, now that machine tools such as precision grinding machines and diamond-turning lathes are being ud to produce optical components often by personnel with a background in mechanical engineering rather than optics. By the same token, mechanical engineering is also beginning to encroach on electronics particularly in the preparation of miconductor substrates.
(1) Mechanical Engineering
One of the earliest applications of diamond turning was the machining of aluminum substrates for computer memory discs, and accuracies are continuously being enhanced in order to improve storage capacity: surface finishes of 3 nm are now being achieved. In the related technologies of optical data storage and retrieval, the tolerances of the critical dimensions of the disc and reading head are about 0.25μm. The tolerances of the component parts of the machine tools ud in their manufacture, i. e. the slide ways and bearings fall well within the nanotechnology range.
Some precision components falling in the manufacturing tolerance band of 5-50nm include gauge blocks, diamond indenter tips, microtome blades, Winchester disc reading heads and ultra precision XY tables (Taniguchi 1986). Examples of precision cylindrical components in two very different fields, and which are made to tolerances of about 100 nm are bearings for mechanical gyroscopes and spindles for video castte recorders.
The theoretical concept that brittle materials may be machined in a ductile mode has been known for some time. If this concept can be applied in practice it would be of significant practical importance becau it would enable materials such as ceramics, glass and silicon to be machined with minimal sub-surface damage 9 and could eliminate or substantially reduce the need for lapping and polishing。
Typically, the conditions for ductile-mode machining require that the depth of out is about 100 nm and that the normal force should fall in the range of 0.l－0.01 N. The machining conditions can be realized only with extremely preci and stiff machine tools，such as the one described by Yoshioka et al （1985），and with which quartz has been ground to a surface roughness of 2 nm peak－to－valley. The significance of this experimental result is that it points the way to the direct grinding of optical components to an optical finish. The principle can be extended to other materials of significan
t commercial importance，such as ceramic turbine blades，which at prent must be subjected to tedious surface finishing procedures to remove the structure－weakening cracks produced by the conventional grinding process.
(2) Optics
In some areas in optics manufacture there is a clear distinction between the technological. approach and the traditional craftsman’s approach，particularly where precision machine tools are employed. On the other hand，In lapping and polishing，there is a large grey area where the two approaches overlap. The large demand for Infrared optics from the 1970s onwards could not be met by the traditional suppliers， and provided a stimulus for the development and application of diamond－turning machines to optic manufacture. The technology has now progresd and the surface figure and finishes that can be obtained span a substantial proportion of the nanotechnology range. Important applications of diamond－turned optics are in the manufacture of unconventionally shaped optics，for example axicons and more generally， aspherics and particularly off－axis components．Such as paraboloids.
The mass production （veral million per annum）of the miniature aspheric lens ud in compac
t disc players and the associated lens moulds provides a good example of the merging of Optics and precision engineering. The form accuracy must be better than 0．2µm and the surface roughness must be below 20nm to meet the criterion for diffraction limited performance.
(3) Electronics
In miconductors，nanotechnology has long been a feature in the development of layers parallel to the substrate and In the substrate surface itlf，and the need for precision is steadily increasing with the advent of layered miconductor structures. [18] About one quarter of the entire miconductor physics community is now engaged in studying aspects of the structures. Normal to the layer surface, the structure is produced by lithography, and for rearch purpos at least, nanometre-sized features are now being developed using x-ray and electron-and ion-beam techniques.
Devices bad on GaAs have captured a significant amount of attention becau they offer the highest digital processing speeds coupled with the additional advantages over silicon of having a higher temperature tolerance and having greater radiation resistance. Quantum-well devices, often made by MBE, typically consist of a multi-layer stack of GaAs interleaved with an alloy such as AlGa
As in which the layers may have thickness from less than 20nm down to near atomic dimensions, to form a superlattice. Other combinations of materials, such as InP and InGaAs can also be ud. Two quantum-well devices are the high electron mobility transistor ( HEMT) and the multiple quantum-well lar, which exploit the hetero-junction between the layers. The lower resistance in the HEMT leads to faster operations. By suitable choice of both the layer thickness and the AI/Ga ratio, it is possible to tailor the frequency of the emitted light I and operation in the visible region is now possible.
5. A look into the Future
With a little imagination, it is not difficult to conjure up visions of future developments in high
