This invention relates to an amorphous or ultra fine-grained, diamond-like material that is substantially free of graphite and hydrogen, and is deposited in a film on a substrate in the form of nanometer-sized, tightly packed nodules of sp.sup.3 -bonded carbon, hereinafter referred to as "nanophase diamond." In one method of preparing the material of the present invention, a laser beam is directed onto the substrate before and/or during deposition of diamond-like particles on the substrate to substantially eliminate graphite particles thereon. Laser energy focused on the substrate is helpful for preventing substantial build-up of graphite. Furthermore, laser energy on the substrate is also useful for preconditioning the substrate to facilitate bonding of diamond-like particles to the substrate.
In recent years, there has been great interest in producing a diamond-like carbon coating for a variety of reasons. First, diamond-like carbon has an extremely hard surface nearly impervious to physical abuse (abrasive or chemical) and is therefore quite useful as a protective surface. Diamond-like carbon is optically transparent (in, e.g., the infrared spectrum), and is therefore believed to be useful in a variety of optics applications such as protecting sensor optical circuits, quantum wells, etc. In addition, diamond-like carbon has been found to have a high electrical resistivity as well as high thermal conductivity--an unusual combination. Diamond-like carbon, when doped, can act as a semiconductor, thereby forming the basis of technology for microcircuitry that can operate under hostile conditions of high temperatures and high radiation levels. Therefore, great interest has been shown in developing techniques for obtaining diamond-like carbon films in commercial quantities for possible use in the semiconductor industry.
While natural diamond is a generally well defined substance, so-called "diamond-like carbon" films are not well defined, possibly because of the many different methods of preparation which contribute unique aspects to the product. From a structural viewpoint, six allotropes of carbon have been identified--two for each of the numbers of dimensions through which the carbon atoms may bond. The two most important carbon allotropes of interest in the optics and semiconductor industries are the two dimensional sp.sup.2 bonding characteristic of graphite, and the three dimensional sp.sup.3 tetrahedral bonding which gives natural diamond its unique properties. The article Low Pressure, Metastable Growth of Diamond and Diamondlike Phases, by John C. Angus and Cliff C. Hayman, 41 Science pp. 1913-21 (August 1988), describes different types of diamond-like phases, methods of production, and possible uses, the disclosure of which is expressly incorporated herein by reference.
Natural diamond has the highest hardness and elastic modulus of any natural material. Diamond is the least compressible natural substance known, has the highest thermal conductivity, and also has a low thermal expansion coefficient. Further, diamond is a wide band gap semiconductor, having a high breakdown voltage (10.sup.7 V/cm) in a saturation velocity (2.7.times.10.sup.7 cm/s) greater than silicon, gallium arsenide, or indium phosphide. Thus, diamond-like carbon films are potentially useful in the electronics industry as a protective, resistive coating with extremely desirable heat sink properties.
Several different kinds of thin films having some of the properties of diamond can be made from pure carbon. Preferably prepared entirely without hydrogen, fluorine or any other catalysts, these materials can be distinguished by their internal structures. Three forms have been reported: (1) defected graphite, (2) i-C, and (3) "amorphic" diamond. See Collins et al., "The Bonding of Protective Films of Amorphic Diamond to Titanium," 71 J. App. Phys. 3260 (1992), the disclosure of which is herein incorporated by reference. The term "amorphic" derives from a combination of the terms "amorphous" and "ceramic," and has become a term of art to describe a material comprising amorphous carbon having diamond-like properties. In the first type of carbon-based material, diamond-like properties seem to accrue from a high density of defects in otherwise orderly graphite that trap electrons and shorten bonds to increase both strength and transparency. The other two materials, i-C and "amorphic" diamond, derive their enhanced characteristics from an abundance of carbon atoms linked by the sp.sup.3 bonds of diamond.
Currently, four major methods are being investigated for producing so-called "diamond-like" carbon films: (1) ion beam deposition; (2) chemical vapor deposition; (3) plasma enhanced chemical vapor depositions; and (4) sputter deposition.
The ion beam deposition method typically involves producing carbon ions by heating a filament and accelerating carbon ions to selected energies for deposit on a substrate in a high vacuum environment. Ion beam systems use differential pumping and mass separation techniques to reduce the level of impurities in the carbon ion fluence to the growing film. While films of diamond-like carbon having desirable properties can be obtained with such ion beam techniques, the films are expensive to produce and are achievable only at very slow rates of growth on the order of 50 angstroms per day to perhaps as high as a few hundred angstroms per day.
The chemical vapor deposition and plasma enhanced chemical vapor deposition methods are similar in operation and have associated problems. Both methods use the dissociation of organic vapors (such as CH.sub.3 OH, C.sub.2 H.sub.2, and CH.sub.3 OHCH.sub.3) to produce both carbon ions and neutral atoms of carbon for deposit on a substrate. Unfortunately, the collateral products of dissociation frequently contaminate the growing film. While both chemical vapor deposition and plasma enhanced chemical vapor deposition achieve film growth rates of practical levels, such films are of poor optical quality and unsuitable for most commercial uses. For example, if the films are amorphous, the internal stress is too high to permit growth to useful thickness; if crystalline, there is no bonding to the substrate. Further, epitaxial growth is simply not possible using chemical vapor deposition techniques.
Sputtering deposition usually includes two ion sources, one for sputtering carbon from a graphite source onto a substrate, and another ion source for breaking the unwanted graphite bonds in the growing film. In the typical sputtering method, an argon ion sputtering gun sputters pure carbon atoms off of a graphite target within a vacuum chamber, and the carbon atoms are condensed onto a substrate. Simultaneously, another argon ion source bombards the substrate to enhance the breakdown of the graphite bonding in favor of a diamond-like sp.sup.3 tetrahedral bond in the growing carbon film. The poor vacuum and relatively high pressure (10.sup.-5 to 10.sup.-4 torr) in sputtering deposition is cumbersome and tends to introduce contamination of the film on a level comparable to those encountered in chemical vapor deposition and plasma enhanced chemical vapor deposition.
Therefore, while many attempts have been made to obtain high quality diamond-like carbon at commercial levels of production, the results have thus far been disappointing. The known methods recited above are deficient in many respects. While the ion beam deposition method produces a good quality film, its slow growth rates are impractical. The chemical vapor deposition and sputter methods are prone to contamination, yielding an unacceptable film in most circumstances. Additionally, the chemical vapor deposition technique creates a growth environment that is too hostile for fragile substrates. Indeed, the aggressive chemical environment characteristic of chemical vapor deposition could result in destruction of the very substrate that the diamond-like film is intended to protect. Moreover, all known methods require elevated temperatures, which often prove impractical if coating an optical substrate is desired. The known methods also involve complex and cumbersome devices to implement.
Chemical vapor deposition techniques have been useful in obtaining diamond-like carbon material denoted "a-C:H," which is an amorphic carbon structure containing a significant amount of hydrogen. In fact, it is believed that hydrogen is necessary for permitting realistic growth of the metastable diamond-like material and that a reduction in the amount of hydrogen below about 20% degrades the film towards graphite. Of course the amount of hydrogen correlates with the proportions of sp.sup.3 to sp.sup.2 bonding and the correlation of diamond-like properties to graphite-like properties.
It appears that some ion-sputter techniques have been successful in producing a diamond-like carbon material without hydrogen--so-called "a-C" film. (See, Low Pressure, Metastable Growth of Diamond and Diamondlike Phases, page 920, supra). Such a-C diamond-like carbon is interesting in that the ratio of sp.sup.3 to sp.sup.2 bonding is believed to be high, with a corresponding increase in diamond-like properties. Unfortunately, the growth rates of such a-C materials are extremely slow and not commercially viable. In addition, internal stresses are too high to permit growth to useful thicknesses. Further, ion energies are too low to permit bonding with the substrates. Apparently, attempts to increase growth rates using increased power densities in such ion sputter techniques resulted in a decrease in the ratio of sp.sup.3 to sp.sup.2 bonding.
Producing diamond-like carbon is just one example of the general problem of producing a layer of material having desirable physical properties where the material is extremely difficult to handle or manipulate. Examples of other such materials include semiconductors, such as silicon, germanium, gallium arsenide, and recently discovered superconducting materials which might be generally characterized as difficult to handle ceramics (e.g., yttrium-barium compounds). Therefore, it would be a significant advance to achieve a method and apparatus which could produce an optical quality diamond-like layer having an abundance of sp.sup.3 bonding (and therefore diamond-like qualities) in commercial quantities. Further, it would be significant if such method and apparatus were useful in producing layers of other types of materials which, using conventional technology, are difficult to handle or produce.