Ultraprecise figuring systems are needed in many scientific fields. For example the mirrors for synchrotron-radiation X-ray facilities and extreme ultraviolet lithography systems should have atomically smoothed surfaces and extremely high figure accuracy of the order of nanometer range. Furthermore, next-generation semiconductor surfaces should be atomically flat as the substrates for nanometer devices. The EEM (Elastic Emission Machining) system developed by the authors can provide the atomically smoothed surfaces free from any crystallographic damage. A basic research has been performed to evaluate the use of a numerically controlled EEM system for ultraprecise figure corrections. We have constructed an EEM figuring system for ultraprecise scientific components. Testing showed that its performances are sufficient for figuring with nanometer accuracy.
A novel chemical machining method using neutral radicals generated by atmospheric-pressure plasma, which we call “chemical vaporization machining” (CVM), has been developed to overcome the problem of deformed layers in conventional mechanical machining processes (turning, grinding, lapping, etc.). By use of a high-speed rotary electrode to generate the plasma we achieved a high machining efficiency equivalent to that of mechanical machining. Since the generated plasma is localized around the electrode due to the high-pressure atmosphere, the spatial resolution is good. The defect density of the machined surface is very low and is equivalent to that of chemical etching. A numerically controlled plasma CVM machine was developed for fabricating ultra-high-accuracy optical elements and semiconductor substrates. Gas bearings are used for the rotary electrode and xy-table to maintain the cleanliness of the atmosphere. A fabricated X-ray mirror had a flatness of 22.5 nm, and a fabricated silicon-on-insulator had a film thickness of only 13.5 nm.
ELID grinding is a method that realizes high quality mirror surface grinding in the nanometer order by use of electrolytically dressing metal-bonded wheels consisting of fine abrasives. Currently, ELID grinding is increasingly being applied as an ultra-precision grinding method to achieve desired surface roughness, high surface accuracy, high surface quality, and high grinding performance. This paper introduces the principle of ELID grinding and discusses some applications of ELID grinding to mirror surface grinding.
Some combinations of different phenomena may cause unexpected yet rather beneficial effects. For instance, the combination of a magnetic field with the action of abrasive against a work material gives rise to a new magnetic field assisted finishing process, which shows the potential to overcome problems with more conventional surface finishing processes. Because magnetic flux flows without being impeded through nonferrous work material, it is possible to influence the abrasive acting force and the abrasive motion against a work surface by controlling the magnetic field. This causes the finishing operation not only on the easily accessible surfaces but also on the areas that are hard to reach by conventional mechanical techniques. This is a distinct advantage of magnetic field assisted finishing. This report presents the finishing principle, characteristics, and mechanism of the magnetic field assisted finishing process. In addition, a few examples of industrial applications are presented.
A definition of super smooth mirror surfaces was given first. On such smooth surfaces the texture as surface roughness is considered to be of the size of surface atoms, provided that there are crystalline terraces but not strain and stress of the damaged layer caused by material processing. Mechanism of material removing in the ultra-precision polishing to reach such a super smooth mirror surface and suitable methods and conditions for the minimization on surface roughness were described, and some examples of such polishing technology were explained: ordinary optical glass surfaces were finished to less than 0.5 nmRy roughness under an improved pitch-polishing. The detail of newly proposed P-MAC (Progressive Mechanical and Chemical) polishing is also described. In this polishing, an abrasive free condition is adopted and the stock removing from work surface accelerated by the chemical reactions accompanying the friction with a closed contact/semi-contact condition between work and polisher surfaces in first step. After removing the ups and downs on surface roughness of work, polishing condition shifts to non-contact condition as second step. By using Br-methanol solution and soft fluorocarbon foam polisher, less than 0.5 nmRy in surface roughness was attained with GaAs wafer surfaces.
The principle of mechanochemical polishing and examples of the method are described. By using the method we can polish hard and functional materials with the abrasive powders that are mechanically softer than the materials. If the powders supplied are chemically reactive with the materials to be polished, mechanochemical reactions take place at the real contact points. Applications of this method to sapphire, silicon wafer, quartz and silicon carbide are shown.
The Ge segregation mechanism in the Si/Ge system has been studied by medium-energy ion scattering (MEIS). One monolayer of Ge was grown on Si(001) at 450oC and annealed at 780oC. We find that the Ge redistributes during the very early stages of the annealing process and remains constant even after annealing at longer periods of time. Therefore, we conclude that the top few layers attain thermal equilibrium in a short period of time (less than 1 min). In addition, we find that the activation energy for the site exchange process occurring at the subsurface region is lower than the previously reported value when Si overlayer is grown on the Ge thin film. The exchange between the second and third layers occurs readily during the growth, even though these layers are buried, and are not expected to reduce the surface free energy. The contribution of the second layer to Ge segregation is similar to that of the topmost layer.
Dynamical observation of Ga adatoms on GaAs(001) surfaces was successfully made by a system using scanning tunneling microscopy(STM) and molecular beam epitaxy(MBE), which were not separated into each chamber but equipped as an incorporate unit. It is found that Ga adatoms were self-organized to a unit cell far from the B-step edge and on the missing dimer row of (2×4) β 2 As-adsorbed surface. Moreover, the three Ga adatoms formed a trimer cluster which changed to a tetramer cluster with addition of one Ga atom. Ga heptamers were also observed. The dangling bonds of a Ga dimer were completely empty, satisfying electron counting heuristics. No clear observation by the filled state imaging indicates that the adatoms are crystalline. However, the Ga trimer clearly observed is metal cluster. The Ga trimer and heptamer have 9 and 21 electrons in the molecular orbital states. The numbers are closed to the magic numbers of spherical jellium, 8 and 20, respectively. This excess electron might become the support between the cluster and the substrate surface, forming 2D magic clusters, the Ga trimer and heptamer, on GaAs(001) surface.