Cleaning and liquid contact with solids – Processes – Including application of electrical radiant or wave energy...
Patent
1994-09-12
1998-12-15
Warden, Jill
Cleaning and liquid contact with solids
Processes
Including application of electrical radiant or wave energy...
134 1, 134 11, 134 12, 156345, 20429816, B08B 700
Patent
active
058490930
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 application of PCT/EP92/03015.
BACKGROUND OF THE INVENTION
The invention relates to a process for cleaning and smoothing the surface of materials having a monocrystalline, polycrystalline or amorphous structure, wherein the surface is polished to the lowest mechanically attainable roughness, and thereafter the surface is bombarded with ions of a defined charge and kinetic energy to remove any impurities thereon.
A standard process of the kind defined above, with variants, for the cleaning and smoothing of surfaces on the level of atomic dimensions is known and is widely applied. This process is based on a publication by H. E. Farnsworth, R. E. Schlier, T. H. George and R. M. Burger in "Journal of Applied Phasics", 1958, volume 29, pages 1150 to 1161. Examples of materials that can be treated are, for instance, silicon monocrystals for the production of integrated circuits; other semiconductors for the production of opto-electronic components; metallic or semiconducting monocrystals for the development of so called nano-structures; monocrystals as substrate for X-ray mirrors made of multilayers; and polycrystalline, or amorphous metallic, semiconducting, or insulating surfaces which have to be atomically clean and smooth for various production processes.
For the known process, the surface of the material is mechanically polished to the smallest possible roughness and then heated in an ultra high vacuum (UHV) up to a temperature near but clearly below the melting point in order to expel from the interior and to evaporate from the surface all easily volatile impurities. After this treatment, the surface is exposed to an ion bombardment with energies between 200 eV and 10000 eV, preferably 5000 eV, and a high flux density between 10 .mu.A/cm.sup.2 and 1000 .mu.A/cm.sup.2 by which binary collisions and collision cascades remove the surface and thereby also the impurities by sputtering. Rare gas ions may be used as the ions, preferably Ar.sup.+ singly charged argon ion (see "Sputtering by Particle Bombardment I", ed. R. Behrisch, volume 47 of "Topics in Applied Physics", Springer, N.Y. 1981 and "Sputtering by Particle Bombardment II", ed. J. Roth, volume 52 of "Topics in Applied Physics", Springer, N.Y. 1983).
The treatment with the known process damages the surface, and the ions used, mostly rare gas ions, penetrate into the sample material so that a follow-up treatment of the sample by various cycles of sputtering and annealing is required, which removes the surface damage and outgases the penetrated rare gas atoms so that clean and, in the case of low index monocrystalline surfaces, flat surfaces can also be obtained. The follow-up treatment by annealing (heating) leads to a polycrystalline material by the formation of microcrystals on a particularly rough surface on an atomic scale and runs the risk of recrystallization of initially amorphous surfaces.
Here the term "clean" is defined with respect to the methods of detection, most of which reach a limit of sensitivity of one impurity atom per 1000 proper surface atoms of the material of interest. During sputtering, the transformation of the kinetic energy of the ions in collisions also creates recoil atoms of the impurities which can penetrate into the sample material. A single cycle thus reduces the impurities on the surface, a fraction of which is, however, injected into the sample material and which reappears at the surface during further erosion. As a result, the number of cycles has to be increased and the surface has to be more deeply eroded when a cleaner surface is required.
It is known to lower the energy of the ions in order to reduce the number of sputtering and heating cycles. On the one hand, this measure reduces the penetration depth of the ions into the material and the implantation of impurities by recoil, but on the other hand, the rate of sputtering and therefore the cleaning efficiency is reduced. (H. Gnaser and H. Oechsner, Surface Science 251-252, 1991, pages 696-700). The reduction of the ion energy has ther
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Markoff Alexander
Warden Jill
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