WO1993005207A1

WO1993005207A1 - Method of nucleating diamond and article produced thereby

Info

Publication number: WO1993005207A1
Application number: PCT/US1992/007439
Authority: WO
Inventors: R. P. H. Chang; Raymond J. Meilunas; Manfred M. Kappes; Shengzhong Liu
Original assignee: Chang R P H; Meilunas Raymond J; Kappes Manfred M; Shengzhong Liu
Priority date: 1991-09-03
Filing date: 1992-09-03
Publication date: 1993-03-18

Abstract

Disclosed is a method of forming a diamond layer on a substrate, especially a nondiamond substrate, wherein diamond nucleation is enhanced by providing a nucleating layer comprising a fullerene or carbon cluster having a geodesic molecular structure on the substrate. The nucleating layer and a carbon-bearing plasma or other gas are then contacted under temperature and pressure conditions effective to nucleate diamond at the nucleating layer. During such contact, the substrate is negatively biased relative to the plasma to impinge positively charged ions in the plasma on the nucleating layer to promote diamond crystallite nucleation.

Description

METHOD OF NUCLEATING DIAMOND

AND ARTICLE PRODUCED THEREBY

The United States Government has rights in this invention pursuant to Contract No. N00014-89-J-1848 between the Office of Naval Research and Northwestern

University and Contract No. DE-FG02-87ER45314 between the U.S. Department of Energy and Northwestern

University, Evanston, Illinois, which contracts grant to

Northwestern University the right to apply for this patent.

FIELD OF THE INVENTION

The present invention relates to formation of a thin diamond layer on a substrate, especially a nondiamond substrate, wherein the nucleation density of diamond crystallites on the substrate is substantially improved.

BACKGROUND OF THE INVENTION

Diamonds are one of the hardest substances known to mankind. But, it also has a plethora of other properties which make it ripe for commercial

exploitation. Diamonds have optical, electrically insulating and heat-transfer capabilities that make it unique. It is an electrical insulator, yet it

dissipates heat faster than silver or copper, thereby allowing a dense packing of electronic components. It has the widest known range of optical transmissivity. It is biocompatible and its frictional properties are akin to "Teflon".

These properties make diamond ideal for use on various products in a number of different technologies. Attention is being directed to products coated with diamond film. For example, diamond coated products have potential use in the electronic, military and aerospace, cutting tool, laser, optical and semiconductor

industries. For instance, diamond film may be used as a protective coating for military and aerospace

applications, such as radomes and infrared sensors, lenses on missile guidance systems and for a variety of optical field equipment requiring protection from rain, dust and sand. Diamond coatings may be used as

protective coating for laser scanning windows, such as at retail-check-out and sunglasses.

Cutting tools, especially those used in the metal working industry, is another product where diamond coated films can potentially be used. Ceramic cutting tool tips or inserts coated with a diamond film can operate at higher speeds, last longer and cost less to manufacture than conventional tools with carbide or synthetic diamond tips or inserts. Other potential uses include metal cutting tools, automated bonding tools, industrial saws and knives, surgical instruments and microtomes.

Furthermore, diamonds could be used to prepare diamond loudspeaker diaphragms for use in loudspeakers. For example, a diamond diaphragm can be prepared by depositing a diamond film on a substrate and then dissolving the substrate. The diamond loudspeaker has properties that exceed that of beryllium, which was known to be the best loudspeaker material heretofore. For instance, the sound propagation speed of the diamond loudspeaker is faster than that of beryllium and its reproducible frequency is greater than that of beryllium

Mechanical parts coated with a protective diamond layer would greatly improve their resistance to wear and tear. Such parts include bearings and watch parts, for example. Prostheses coated with diamond film can enjoy a long service life because the diamond film coating enhances smoothness, chemical inertness, hardness and biocompatibility.

Diamond is a very desirable material in the optical industry and in lasers. For example, diamond film coated lens have been used for focusing laser beams.

Diamond coated film can be potentially used as a semiconductor. Chips made of single crystal diamond runs faster and cooler than those made of silicon or gallium arsenide.

In addition, diamond film can be used as thermistors. Thermistors made of polycrystalline diamond film can operate at temperatures much greater than those made up of other material, such as silicon, gallium arsenide or silicon carbide.

Other potential uses for the diamond film include transistors as well as light emitting devices (LED'S).

As a result of the many promising applications for thin diamond layers in the fabrication of protective coatings, optical coatings, electronic devices, etc., considerable effort has been expended in developing low pressure, low temperature processes for the deposition of these layers on various substrates, especially nondiamond substrates that would be involved in such applications. Representative of such low pressure thin diamond film deposition methods are chemical vapor deposition (CVD), plasma enhanced chemical vapor

deposition (PECVD), hot filament chemical vapor deposition (HFCVD), oxygen-acetylene torch, and plasma torch techniques.

A severe drawback experienced in practicing these thin diamond film deposition processes has involved the need to pretreat nondiamond substrates in a manner to provide a sufficient density of diamond nucleation sites on the substrate surface to enable subsequent growth of a continuous diamond layer. The most common pretreatment developed to-date to achieve the required diamond nucleation density involves

abrading the substrate surface with diamond powder prior to conducting the low pressure deposition process. The Iijima et al. technical article "Early Formation of Chemical Vapor Deposition Diamond Films", Appl. Phys. Lett., 57, p. 2646 (1990) attributes the efficacy of the diamond powder abrading pretreatment in enhancing diamond crystallite nucleation on the substrate to the seeding action provided by residual "diamond dust" particles present on the substrate surface following the pretreatment. Unfortunately, the diamond powder

abrading pretreatment constitutes a severe processing limitation for many potential applications where the diamond layer will be nucleated and grown on a

nondiamond substrate and where large and/or non-planar substrate surfaces are involved.

Other pretreatments for enhancing diamond nucleation on nondiamond substrates have involved application of pump oil, diamond-like carbon films

(DLC), and even fingerprints to the substrate surface. In addition, DC voltage biasing of the substrate has been employed to this same end. However, none of these techniques has been reported as providing significant nucleation enhancement during the thin diamond film deposition processes involved.

An object of the present invention is to provide a low pressure diamond deposition method using a novel nucleating layer on a substrate to substantially improve diamond crystallite nucleation and overcome the limitations of the diamond powder abrading technique described hereinabove. Anther object of the present invention is to provide an article comprising a

substrate on which a diamond layer is nucleated and grown at a novel nucleating layer provided on the substrate.

SUMMARY OF THE INVENTION

The present invention contemplates a method of forming a diamond layer on at least a portion of a substrate, especially, a nondiamond substrate, wherein diamond nucleation is enhanced by providing on at least a portion of the substrate a nucleating layer comprising a carbon cluster having a geodesic molecular structure (i.e., a molecular structure comprised of an array or grid of polygons) and contacting the nucleating layer with a carbon-bearing gas under temperature and pressure conditions effective to nucleate diamond at the

nucleating layer. The nucleating layer preferably comprises a fullerene molecule, e.g., C₇₀ fullerene, having a combination of hexagons and pentagons joined at their vertices.

In one embodiment of the invention, the nucleating layer is deposited (e.g., sublimated, sputtered, etc.) on the substrate to thickness of about 100 to about 2000 angstroms. The nucleating layer may be deposited as a continuous layer on a substrate surface or as one or more discrete regions on the substrate surface so as to selectively nucleate diamond crystallites at the region(s).

In still another embodiment of the invention, the carbon-bearing gas comprises a mixture of hydrogen and a hydrocarbon. The carbon-bearing gas preferably comprises a carbon-bearing reducing plasma wherein hydrogen and a hydrocarbon are ionized.

In a still further embodiment of the invention, the nucleating layer is impinged by particles in a pretreatment operation to promote diamond

nucleation. The pretreatment operation may occur prior to and/or concurrently with a nucleation stage of the diamond deposition process.

In a particular working embodiment of the invention, diamond crystallite nucleation is enhanced by forming on at least a portion of the substrate a

nucleating layer comprising a C₇₀ fullerene, or a portion of its molecular structure, and contacting the nucleating layer and a carbon-bearing plasma while the substrate is electrically biased at a negative potential relative to the plasma to accelerate ions in the plasma to impinge on the nucleating layer to facilitate diamond nucleation.

The present invention also contemplates an article comprising a substrate and a diamond layer nucleated and grown on at least a portion of the

substrate having the aforementioned nucleating layer thereon. DESCRIPTION OF THE DRAWINGS

The aforementioned objects and advantages of the invention will become more fully apparent from the detailed description and drawings which follow.

Figure 1 is a schematic view of a microwave plasma enhanced, low pressure chemical vapor deposition (CVD) apparatus for practicing an embodiment of the method of the invention.

Figure 2 is an enlarged schematic view of the substrate holder of the apparatus of Figure 1.

Figures 3a, b, c are micrographs of diamond crystallites nucleated and grown on circular nucleating layer dots on a silicon substrate in accordance with Example 1 set forth hereinbelow.

DETAILED DESCRIPTION OF THE INVENTION

As indicated hereinabove, the present invention is directed to a method for enhancing diamond nucleation on surfaces. The enhancement of diamond nucleation on surfaces consists of the following

requirements: (1) the need for steps or ledges on the order of several tens of angstroms or less (on a

substrate); (2) the presence of a structured carbon source to act as diamond seeds; (3) a means whereby gas phase nucleation can readily take place on the seeds; (4) the need for a structural carbon source to be air stable and withstand the environment in which plasma enhanced chemical vapor deposition of diamond takes place and (5) to have a means of initiating diamond nucleation on the carbon seeds during CVD processes. Ideal surfaces which meet the above requirements include carbon clusters having a geodesic molecular structure and/or films. Fullerene is a type of carbon cluster having a geodesic molecular structure. These molecules form cages having a central cavity. These molecules can take the stable form of hollow closed nets composed of 12 pentagons and at least one hexagon. In stable

fullerenes, all pentagons are fully annelated by

hexagons. Examples include buckminsterfullerene (C₆₀), C₇₀ fullerene, C₇₆ fullerene, C₇₈ fullerene, C₈₂

fullerene, C₈₄ fullerenes, C₉₀ fullerenes, C₉₂

fullerenes, C₉₆ fullerenes and the like. However, larger fullerenes in the range between C₉₆ and C₂₅₀ can also be used, for example, C₁₂₀ fullerenes, C₂₄₀

fullerenes and the like. In fact, giant fullerenes with molecular formulas up to C₄₀₀ and beyond, can be

utilized. These include carbon clusters in the C₆₀₀- C₇₀₀ range. Another fullerene that could be used include buckytubes, i.e., fullerenes containing micron long concentric needle-like tubes in which the hexagons are arranged in a helical pattern. See, Iijima, S.,

Nature 1991, 354, 56-58, the contents of which are incorporated by reference. All of these fullerenes can be used as the nucleating layer.

However, the preferred carbon clusters are those even-numbered fullerenes having from 60 to about 520 carbon atoms. More preferred are those wherein the number of carbon atoms range from 60 to about 120, and especially preferred in the carbon cluster ranging from 60 to about 100 carbon atoms. The buckytubes are also preferred.

The use of carbon clusters in the methodology of the present invention is illustrated by reference to the drawings in Figures 1-3. Figures 1-2 schematically illustrate a

microwave plasma enhanced chemical vapor deposition (CVD) apparatus for practicing one embodiment of the method of the invention. The microwave plasma enhanced CVD apparatus is described by R. Meilunas and R.P.H. Chang in Proceedings of the 2nd ICEM Conference,

Materials Research Society, Pittsburgh, PA., page 609 (1990), the teachings of which are incorporated herein by reference.

in practicing a method embodiment of the invention using the plasma enhanced CVD apparatus illustrated in Figures 1-2, microwave energy at a frequency of 2.45 GHz is transmitted from a 1 KW

generator 10 to a cylindrical, cold wall, stainless steel vacuum chamber 12 via a mode converter 14

comprising a rectangular, metal waveguide 16 and

circular, metal waveguide 18. A water cooled circulator 20 is positioned between the microwave generator 10 and the vacuum chamber 12 to protect the generator from any unwanted reflected power transmitted back from the vacuum chamber 12. Any reflected power is diverted by the circulator 20 to a water cooled dummy load 22. A four stub tuner 24 is employed to impedance match the deposition system to the generator 10, thereby

maximizing the forward power transmitted into the plasma P established in the vacuum chamber 12. The mode converter 14 is employed to alter the electric and magnetic fields from the rectangular mode to a circular mode such that the electric field lines of the

propagating microwave energy are circularly symmetrical relative to the longitudinal axis of the vacuum chamber 12, thereby centering the plasma P in the vacuum chamber 12.

The microwave energy is transferred into a deposition region DP of the vacuum chamber 12 through a high purity quartz window 26 mounted on a flange 12a of the vacuum chamber 12. The deposition region DP of the vacuum chamber 12 has dimensions matched to the

dimensions of the circular waveguide 18 to minimize abrupt junctions at the interface of the vacuum chamber 12 and the waveguide 18 as power attenuation in the vacuum chamber 12.

Referring to Figure 1, ultra-high purity hydrogen gas and ultra-high purity hydrocarbon (e.g., methane) gas are supplied to the deposition region DP via a common conduit 33 communicating with respective gas supply conduits 34, 36. The gas supply conduits 34, 36 extend from conventional gas sources 38, 40 (e.g., ultra-high purity hydrogen and methane gas cylinders). Hydrogen flow and hydrocarbon flow to the deposition region DP are controlled by a mass flow controller (not shown) in the respective supply conduits 34, 36. The metered gas flows are mixed in the common conduit 33 to provide desired hydrogen/methane gas mixture ratios in the deposition region DP during the nucleation stage and the growth stage of the plasma enhanced CVD process as will be described hereinbelow. Before the

hydrogen/methane gas mixture is introduced into the deposition region DP, a vacuum pump 42 is actuated to evacuate the chamber 12 to a base pressure of about 2 x 10^-6 torr. The vacuum pump 42 communicates to the chamber 12 via a pressure control valve 44, such as a gate valve, in a conduit 46. A thin diamond layer is formed on a suitable substrate 50 which is located on a substrate holder mechanism 53 that includes a tubular quartz substrate support 54 and an annular graphite cover 56 overlying the substrate 50 at the upper end 54a of the support 54. The support upper end 54a is sealed in a flat, gas-tight manner to provide a support platform for the substrate 50 and to prevent ingress of contaminating gases from the ambient atmosphere external of the vacuum chamber 12. The cover 56 is biased downwardly by a plurality of springs 58 (two shown) connected between quartz rods 59 on the underside of the collar 56 and the bottom wall 12b of the vacuum chamber 12. The collar 56 thereby clamps the substrate 50 on the sealed upper end of the support 54. The quartz rods 59 electrically isolate the substrate 50 and the cover 56 from ground potential.

The substrate support 54 is movable by a linear position 61 to enable desired positioning of the substrate surface relative to the plasma P.

A strip 63 of platinum foil is clamped between the substrate 50 and the cover 56 in electrical contact therewith in order to electrically bias the substrate relative to the plasma P in accordance with a feature of the invention to be described hereinbelow. The foil strip 63 is spot welded to a platinum wire 62 which is connected to and passes through a vacuum feedthrough 64 in the chamber wall 12c. The wire 62 is connected to the negative terminal of an external direct current voltage source 66 as shown in Figure 1. The other terminal of the voltage source 66 as well as the vacuum chamber wall 12b are connected to ground as also shown in Figure 1. A wire mesh microwave attenuation tube (not shown) is employed about the foil strip 63 and wire 62 between the cover 56 and the feedthrough 64. The attenuation tube has a diameter below the cutoff

frequency for transmission of a 2.45 GHz circular wave of any of its higher order modes. The attenuation tube thus functions to attenuate any complex waveform that might travel out of the deposition region DP down the electrical connection (foil strip 63 and wire 62).

In accordance with the present invention, a novel nucleating layer 60 is provided on the substrate surface to substantially enhance the density of diamond crystallites nucleated on the substrate. The nucleating layer 60 can be deposited on a discrete region or portion of the substrate surface where diamond is to be selectively nucleated and grown. Alternately, the nucleating layer 60 can be deposited as a continuous layer on the substrate surface to form a corresponding continuous diamond layer or film on the surface; for example, for use a protective layer on the substrate.

The nucleating layer 60 is effective to enhance diamond crystallite nucleation on a variety of nondiamond substrate materials including, but not limited to, metals such as Mo, semiconductors such as silicon, and insulators such as silicon dioxide.

Specific substrate materials used in practicing the invention are described in the Examples set forth in detail hereinbelow.

As indicated hereinabove, the nucleating layer 60 deposited on the substrate surface comprises a carbon cluster containing a geodesic molecular structure.

The nucleating layer 60 deposited on the substrate surface preferably comprises a C₇₀ fullerene whose molecular structure comprises, as is known, a combination of hexagonal molecular units (or faces) and pentagonal molecular units (faces) arranged in a pattern or array and joined at vertices of the units to form a hollow, oblong ball-shaped molecule. Carbon atoms are located at the vertices of the joined hexagons and the pentagons. The molecular structure of the C_{7 0} fullerene is characterized and illustrated in Popular Science, August, 1991, pp. 52-57 and 87 as well as other

technical literature including J. Am. Chem. Soc. 113,

(1991) p. 3619, the teachings of which are incorporated herein by reference.

The C₇₀ fullerene is obtained from carbon soot prepared in accordance with a known preparation

technique wherein an electrical arc is struck between opposing graphite electrodes in a helium atmosphere to vaporize the graphite electrode material. The desired C₇₀ fullerene is solvent extracted and

chromatographically separated from other fullerenes. Such a preparation technique is described in Nature, Vol. 347, 1990, p. 354 as well as other literature referred to therein and also in the technical article entitled "Infrared and Raman Spectra of C₆₀ and C₇₀ Solid Films at Room Temperature", J. Appl. Phys. 1991, the teachings of both of which are incorporated herein by reference. The presence of the C₇₀ fullerene is confirmed by electron impact mass spectroscopy, IR and Raman spectroscopy as described in that technical article.

In another embodiment, the nucleating layer may also comprise a C₆₀ fullerene whose molecular structure comprises, as is also known, a combination of hexagonal molecular units (or faces) and pentagonal molecular units (or faces) arranged and joined at vertices of the units to form a hollow, spherical ballshaped molecule wherein the carbon atoms are located at the vertices of the joined hexagons and pentagons. The molecular structure of the C₆₀ fullerene is also

characterized and illustrated in Popular Science,

August, 1991, pp. 52-57 and 87 as well as other

technical literature including Chem. Phys. Lett. 179, (1991), p. 181. The C₆₀ fullerene is prepared in the same manner described hereinabove for the C₇₀ fullerene. Other members of the fullerene family of molecules, such as C₇₂, C₇₆, C₈₄, C₉₀, C₉₂, C₉₆ and like, that exhibit an appropriate ordered molecular structure, stability in air, and ability to withstand, at least to some degree, the environment in which plasma enhanced CVD is conducted may also find use as a nucleating layer in practicing the invention.

Preparation of C₇₆ , C₇₈ and C₈₄ is described by Diederich, et al. in Science, 1991, 252, 548-551 and by Ettl, et al. in Nature, 1991, 353, 149-153 and by Diederich, et al. in Ace. Chem. Res., 1992, 119-126, the contents and references cited therein which are

incorporated herein by reference. These fullerenes may be prepared as described above for the C₇₀ fullerenes. The various fullerenes are separated by column

chromatography using alumina as the adsorbent and using hexane/toluene (95:5) as the eluent, followed by several HPLC runs on C₁₈ reversed column using acetonitrile/ toluene (1:1) as the eluent. C₉₀ , C₉₂ and C₉₆ are prepared by similar techniques. Buckytubes is produced using an arc-discharge evaporation method similar to that used for the

synthesis of other fullerenes. Its preparation is described by Iijima, S., in Nature, 1991, 354, 56-58, the contents of which are incorporated by reference.

Moreover, mixtures of C₆₀, C₇₀ and other fullerenes may be employed as the nucleating layer 60. For example, the carbon soot referred to hereinabove in the production of C₇₀ fullerene may itself (i.e., the soot) be used as the nucleating layer.

The invention envisions the substitution and/or addition of one or more elements and/or radicals at one or some of the carbon atom positions of a carbon cluster molecule, e.g., C₇₀ fullerene, so as to enhance its efficacy as a nucleating layer in the low pressure CVD process. For example, silicon may be substituted for one or more of the carbon atoms of the C₇₀ fullerene molecule or other fullerene molecules. In addition, oxygen, hydrogen or halogen (e.g., fluorine) may be added to one or more carbon atoms of the C₇₀ fullerene molecule or other fullerene molecule to form oxides, hydrides or fluorides of the fullerene. See, for example, Selig, et al., JACS, 1991, 113, 5475; Olah, et al., JACS, 1991, 113, 9387 and Tebbe, et al., JACS,

1991, 113, 9900. Metal complexes of carbon clusters, e.g., alkali metal, Group VIII metals (e.g., platinum, palladium, nickel and the like) and other

metallofullerenes, such as lanthanum or scandium

containing fullerenes, can also be used. Other

fulleroids, in which dipolar molecules are added to the fullerene can also be used. Examples of fulleroids include 4, 4-dibromodiphenyl fulleroid, bis-(4- bromophenyl)fulleroid, bis-4-n-dimethylaminophenyl fulleroids, bis-(4-methoxyphenyl)fulleroids, bis-4- methylphenyl) fulleroids and the like.

The nucleating layer 60 may be deposited on the substrate surface by various techniques. For example, an appropriate quantity of the carbon cluster, e.g., C₇₀ or C₆₀ fullerene material, can be positioned in a conventional sublimation chamber at 10^-6 torr and heated by a hot filament to a sufficiently high

temperature (e.g., 600°C for C₇₀ and 500°C for C₆₀) to sublimate the material as a thin layer onto the

substrate surface appropriately located relative to the fullerene material in the chamber. For example, the substrate surface is typically located about two (2) inches above and facing the heated fullerene material in the sublimation chamber so that evaporated material deposits on the substrate surface as a thin layer.

Typically, the nucleating layer deposited on the substrate surface will have a thickness of about 100 to about 2000 angstroms. A specific thickness of the nucleating layer used in practicing the invention are described in the Examples set forth hereinbelow.

Alternately, the nucleating layer may be formed in the vacuum chamber 12 and deposited on the substrate surface in the deposition region DP shown in Figure 1. For example, a graphite source (not shown) may be positioned in the vacuum chamber 12 so as to be impinged by a laser beam or particle (ion) beam

generated by a suitable beam generator (not shown) also disposed in the vacuum chamber 12. The beam is impinged on the graphite source under conditions to sputter carbon atoms therefrom for recombination as fullerene molecules that are deposited onto the substrate surface. This sputtering technique is advantageous to eliminate the need to deposit the nucleating layer 60 in a

separate sublimation step outside the vacuum chamber 12.

As mentioned hereinabove, the nucleating layer may be deposited as one or more discrete regions on the substrate surface or as a continuous layer thereon. The capability of depositing the nucleating layer at

discrete regions renders the invention useful in

lithography techniques employed in the electronics industry to fabricate microelectronic devices. The diamond nucleated at one or more of these discrete regions can be grown to form an integrated circuit component, such as an interconnect, heat sink, etc. at appropriate locations on a semiconductor wafer or other microelectronic device substrate.

Nucleation and growth of diamond at the nucleating layer are effected by contacting the plasma P established in the deposition region DP of the vacuum chamber 12 and the nucleating layer under conditions of temperature, pressure, gas mixture composition, gas flow rate, etc. selected to this end.

In accordance with one embodiment of the invention, a pretreatment of the nucleating layer to promote diamond nucleation is conducted concurrently with a nucleation stage of the low pressure CVD process. The pretreatment/nucleation stage is conducted using a plasma P rich in hydrocarbon as compared to the plasma used during the growth stage of the CVD process . For example, a typical gas mixture supplied to the

deposition region DP during the pretreatment/nucleation stage comprises about 3-20 volume % and more preferably 5-15 volume % methane and the balance hydrogen. The gas mixture is supplied at 100 seem (standard cubic

centimeters per minute) to 15 torr total pressure in the chamber 12. On the other hand, a typical gas mixture supplied to the deposition region DP during the growth stage comprises about 1 volume % and the balance

hydrogen. The gas mixture is supplied at about 100 sccm to a 100 torr total pressure in the chamber 12. The substrate 50 typically is heated by direct interaction with the plasma P and microwave induction heating to a temperature of about 30°C to 500°C during the

pretreatment/nucleation stage and to about 700°C to about 950°C during the growth stage. Alternately, a separate heating device may be employed in the chamber 12 to heat the substrate to the desired temperature.

During the pretreatment/nucleation stage, the substrate is electrically biased to accelerate

positively charged particles (cations) in the

hydrocarbon-rich plasma P to impinge on the nucleating layer 60 while it is in contact with the plasma. In particular, the substrate 50 is biased negatively from about 100 to about 300 volts relative to the plasma (and ground potential) by the voltage source 66 shown in Figure 1. The impinging positive ions from the plasma P are believed to cleave carbon-carbon bonds of the nucleating layer and thereby create sites for gas phase carbon species to nucleate, although Applicants do no wish to be bound by this explanation. Diamond appears to selectively nucleate at the nucleating layer at some time following initial cation impact. The nucleating layer remains in contact with the plasma for a period of time to initiate sufficient diamond crystallite nucleation to enable subsequent growth to a continuous diamond layer. To this end, a typical duration of the pretreatment/nucleation stage is on the order of several minutes (e.g., about 5-15 minutes).

In lieu of using positive ions from the plasma to break-up the carbon bonds of the carbon cluster molecule, e.g., C₇₀ , C₆₀ , and the like, carbon or hydrocarbyl ions from an ion source in the chamber 12 may be used. Other alternative methods include inert ions (such as argon, neon, helium, etc.), laser beam, electron beam, or energetic neutral carbon/or inert gas beams in a medium of carbon carrying gas such as CH₄ or CH₄ during pretreatment.

Following the pretreatment/nucleation stage, growth of the diamond crystallites nucleated at the nucleating layer 60 is then initiated in the region DP under conditions of temperature, pressure, gas mixture composition, gas flow rate, etc. described hereinabove while the substrate is at a floating potential (i.e., the electrical biasing is discontinued during the growth stage). Such diamond growing conditions are reported by R. Meilunas and R.P.H. Chang in the aforementioned technical article in Proceedings of the 2nd ICEM

Conference (1990) and are also set forth hereinbelow in the Examples. The growth stage is conducted for a time period (e.g., 60 minutes) to form a continuous

polycrystalline diamond layer or film having a thickness in the range of about 1 micron to about 10 microns. The grown diamond layer exhibits a grain size less than approximately one (1) micron.

The following Examples are offered to further illustrate, but not limit, the present invention. EXAMPLE 1 - Nucleation/Growth of Diamond

on Silicon Substrates

Semiconductor grade silicon substrates as received from the manufacturer were used. Native oxide on the silicon substrates can be removed (optional) by dipping in buffer (or dilute) HF acid. A pure C₇₀ nucleating film of one thousand angstrom thickness was sublimated at about 600°C at 10^-6 torr in a conventional thermal evaporator onto each substrate. During

sublimation, a shadow mask can be used, if desired, to lithographically define the areas where C₇₀ is

sublimated on the substrate. In a typical shadow mask that was used, the areas comprise circular dots each 200 microns in diameter.

Pretreatment/Nucleation: Each silicon substrate with the C₇₀ film thereon was loaded into the plasma enhanced chemical vapor deposition machine described above for nucleation and growth. To activate the film for diamond nucleation, a pretreatment of positive ion bombardment by biasing (at 200 volts) the substrate with respect to the plasma was applied. During the pretreatment, which typically was conducted for 15 minutes, the gas

composition was 10% CH₄ in H₂ of 15 torr total pressure, microwave power 400 Watts and a gas flow rate of 100 seem. The substrate temperature was 400°C. Such a treatment initiated massive diamond nucleation at the nucleating film (i.e., at the circular dots) as

evidenced by Raman measurements of a silicon substrate surface after this pretreatment/nucleation which

indicated the presence of nanocrystalline diamond on the surface. Diamond crystallite growth was then initiated in region DP on the pretreated/nucleation substrate in a growth stage lasting about 60 minutes using standard conditions; e.g., substrate temperature 900°C; 1% CH₄ in H₂; total pressure of 100 Torr; microwave power 800 Watts; gas flow rate of 100 seem.

Figures 3a, 3b, 3c are micrographs at different magnifications of continuous diamond film growth observed at the circular nucleating dots on a silicon substrate surface. In Figure 3d the Raman spectra of the diamond film that was grown is shown. The selective nucleation and growth of diamond at the nucleating dots (as compared to the silicon substrate) is evident.

EXAMPLE 2 - Nucleation/Growth of Diamond on SiO₂

Instead of a silicon substrate, a quartz substrate was used in this example. The same conditions of sublimation of the C₇₀ nucleating film on the

substrate and for pretreatment/nucleation and growth of diamond were used as described for Example 1. Diamond nucleation and growth similar to that described for Example 1 were observed.

EXAMPLE 3 - Nucleation/Growth of Diamond on Molybdenum Surface

Instead of a silicon substrate, a molybdenum substrate was used in this example. Removal of the molybdenum oxide surface prior to C₇₀ sublimation is optional. The same conditions for sublimation of the C₇₀ nucleating film and for pretreatment/nucleation and growth of diamond were used as described in Example 1. Diamond nucleation and growth similar to that described for Example 1 were observed.

EXAMPLE 4 - Nucleation/Growth of Diamond on Silicon Substrate Using C₆₀ Nucleating layer

In this example, a C₆₀ nucleating layer was sublimated on a silicon substrate in a manner described hereinabove. The same conditions for pretreatment/ nucleation and growth of diamond were used as in Example 1. Diamond nucleation and growth on the C₆₀ dots was observed to be a few orders of magnitude less than that observed in Example 1.

EXAMPLE 5 - Nucleation/Growth of Diamond on Silicon Substrate Using Carbon Soot Nucleating Layer

In this example, the aforementioned carbon soot comprising a mixture of C₇₀, C₆₀ and possibly other fullerenes was employed as the nucleating layer. The same conditions of pretreatment/nucleation and growth of diamond were used as in Example 1. Diamond nucleation and growth on the circular dots was observed to be a few orders of magnitude less than that observed in Example 1.

In lieu of the concurrent pretreatment/ nucleation stage described hereinabove, an alternate embodiment of the invention envisions pretreating the nucleating layer 60 prior to conducting the low pressure CVD process such that only a portion of the fullerene molecular structure remains on the substrate as a nucleating layer. For example, a nucleating layer 60 comprising, or having an outer region comprising, a fractional portion of the fullerene molecule (for example, C₆₀ , C₈₄ and more preferably C₇₀ , and the like) may be formed by ion beam, laser beam or intense

electron beam impingement of the nucleating layer in a separate pretreatment step prior to low pressure CVD. The pretreated substrate is then placed in the vacuum chamber 12 for nucleation and growth of diamond at the nucleating layer by the plasma enhanced CVD process described hereinabove with or without negative substrate biasing during the nucleation stage of the process.

The term "carbon cluster having a geodesic molecular structure" as used in the appended claims is intended to include the aforementioned C₇₀ fullerene, C₆₀ fullerene, other fullerenes or carbon clusters having a molecular structure comprised of an array or grid of polygons and that are effective to enhance diamond nucleation in low pressure diamond deposition processes, as well as mixtures of such molecules (e.g., the aforementioned carbon soot). The term is also intended to include portions of such molecules that, for example, may remain on the substrate surface after the pretreatments described in the preceding paragraph.

Moreover, the term is intended to include substitution/ addition modified forms of such molecules, or portions thereof, wherein one or more elements and/or radicals are substituted and/or added at one or some carbon atoms positions of the molecular structure.

The term "reducing gas" as used herein and in the claims refers to the presence of hydrogen gas.

"Gas," as used herein, refers to a molecule or atom in the gaseous state at standard temperatures and pressures. It also includes those molecules or atoms which are volatile liquids at 1 atm pressure and room temperature. It includes the neutral molecule or atom as well as the plasma. The neutral molecule or atom, as defined herein, refers to the molecule or atom in the unexcited state (i.e., the molecule or atom itself).

The term "neutral molecules or atoms" also include, however, the excited molecules or atoms, (the molecule or atom in an excited state) and radicals thereof. It also includes the charged species and electrons (ions) of the unexcited or excited molecules or atoms.

"Plasma" as used herein refers to a neutral mixture of positively and negatively charged particle interacting with an electromagnetic field.

The term "carbon bearing gas" as used herein is intended to comprise carbon containing molecules or atoms which are gases at standard temperatures and pressures or which are volatile liquids at 1 atm

pressure and room temperature.

It is preferred that the carbon containing molecules or atoms are hydrocarbons or oxygen containing hydrocarbons or the halogenated containing hydrocarbons. The volatile liquids are preferably organic solvents, for example, hydrocarbon as well as aromatic solvents, ethers, esters, hexanes, alcohols and fluorinated and chlorinated hydrocarbons. It is preferable that the carbon containing molecule or atom is a gas at standard pressure and temperature. It is more preferable that the carbon containing molecule or atom is a hydrocarbon, and more preferably aliphatic. It is most preferable that the carbon containing molecule or atom is a

hydrocarbon and a gas at standard temperature and pressure. It is also preferable that the carbon

containing molecule or atom contain no more than 10 carbon atoms and most preferably no more than 8 carbon atoms and most preferably no more than 4 carbon atoms. Examples include carbon monoxide, carbon dioxide, hydrocarbons, (e.g., methane, ethane, propane, butane, pentane, hexane, heptanes, octanes, cyclopentane, cyclohexane, petroleum ether and the like), halogenated hydrocarbons, (e.g., carbon tetrachloride, carbon tetrafluoride, methylene, chloride, methylene fluoride, chloroform, fluoroform, methyl chloride, methyl

fluoride, and the like), alcohols (e.g., methanol, ethanol, propanol, butanol, and the like), ethers,

(e.g., diethyl ether, dimethyl ether, methyl ethyl ether and the like), ketones (e.g., acetone, and the like), ester (e.g., methyl acetate, ethyl acetate, and the like), aromatics, (e.g., benzene, toluene, ethyl

benzene, and the like), carbon dioxide and carbon monoxide. Preferred examples include the hydrocarbons, especially those containing 1-4 carbon atoms, the halogenated hydrocarbons, especially the methyl

halogenated compounds, alcohols, especially the methyl and ethyl alcohols, ethers, carbon dioxide and carbon monoxide. More preferred examples include the

hydrocarbons which are gases at standard temperature and pressure and carbon monoxide and carbon dioxide. The most preferred are the hydrocarbons which are gases at STP.

The term "carbon bearing plasma" is intended to include the charged species of the carbon bearing gas as defined herein.

Although the invention has been described hereinabove as being practiced using a plasma enhanced, low pressure CVD apparatus/process, the invention is not so limited and may be practiced using other thin diamond film deposition apparatus/processes including, but not limited to, hot filament CVD, non-plasma enhanced CVD, gas torch, plasma torch and laser ablation.

Moreover, while the invention has been

described in terms of specific embodiments thereof, it is not intended to be limited thereto but rather only to the extent set forth hereafter in the following claims.

Claims

We claim:

1. In a method of forming a diamond layer on at least a portion of a substrate, the improvement for enhancing diamond nucleation comprising the steps of: a) forming on at least a portion of the substrate a nucleating layer comprising a carbon cluster having a geodesic molecular structure, and b) contacting the nucleating layer with a carbon-bearing gas under temperature and pressure conditions effective to nucleate diamond at said nucleating layer.

2. The method of Claim 1 wherein the nucleating layer is deposited on at least a portion of a non-diamond substrate.

3. The method of Claim 1 wherein the nucleating layer comprises C₇₀ fullerene.

4. The method of Claim 1 wherein the nucleating layer comprises a portion of the molecular structure of C₇₀ fullerene.

5. The method of Claim 4 wherein the portion of the C₇₀ fullerene molecule comprises six hexagons and four pentagons thereof.

6. The method of Claim 1 wherein the nucleating layer comprises C₆₀ fullerene.

7. The method of Claim 1 wherein the nucleating layer is deposited on the substrate to thickness of about 100 to about 2000 angstroms.

8. The method of Claim 1 wherein in step (a), the nucleating layer is deposited on a discrete region of the substrate.

9. The method of Claim 8 wherein in step (b), the nucleating layer and the carbon-bearing gas are contacted to selectively nucleate diamond at said region.

10. The method of Claim 9 wherein the diamond nucleated at said region is grown to form a component of an electrical device.

11. The method of Claim 1 wherein in step (a), the nucleating layer is deposited as a continuous layer on the substrate.

12. The method of Claim 1 wherein in step (b), the nucleating layer and a carbon-bearing reducing gas are contacted.

13. The method of Claim 12 wherein the carbon- bearing reducing gas comprises a mixture of hydrogen and a hydrocarbon.

14. The method of Claim 1 wherein in step (b), a carbon-bearing gas is a carbon-bearing plasma.

15. The method of Claim 1 which further comprises a step after step (a) and before step (b), of impinging particles on the nucleating layer.

16. The method of Claim 1 which further comprises impinging particles on the nucleating layer during step (b).

17. In a method of forming a diamond layer on at least a portion of a substrate, the improvement for enhancing diamond nucleation comprising the steps of: a) forming on at least a portion of the substrate a nucleating layer comprising a carbon cluster having a geodesic molecular structure, and b) contacting the nucleating layer with a carbon-bearing plasma while the substrate is electrically biased at a negative potential relative to said plasma by an

external power source connected to said substrate so as to nucleate diamond at said nucleating layer.

18. The method of Claim 17 wherein the nucleating layer is deposited on at least a portion of a non-diamond substrate.

19. The method of Claim 17 wherein the nucleating layer comprises C₇₀ fullerene.

20. The method of Claim 17 wherein the nucleating layer comprises a portion of the molecular structure of C₇₀ fullerene.

21. The method of Claim 20 wherein the portion of the C₇₀ fullerene comprises six hexagons and four pentagons thereof.

22. The method of Claim 17 wherein the nucleating layer comprises C₆₀ fullerene.

23. The method of Claim 17 wherein the nucleating layer is deposited on the substrate to a thickness of about 100 to about 2000 angstroms.

24. The method of Claim 17 wherein in step (a), the nucleating layer is deposited on a

discrete region of the substrate.

25. The method of Claim 24 wherein in step (b), the nucleating layer and the carbon-bearing gas are contacted to selectively nucleate diamond at said region.

26. The method of Claim 25 wherein the diamond nucleated at said region is grown to form a component of an electrical device.

27. The method of Claim 17 wherein in

step (a), the nucleating layer is deposited as a

continuous layer on the substrate.

28. The method of Claim 17 wherein in step (b), the nucleating layer and a carbon-bearing reducing plasma are contacted.

29. The method of Claim 28 wherein the carbon- bearing reducing plasma comprises ionized hydrogen and ionized hydrocarbon.

30. In a method of forming a diamond layer on at least a portion of a nondiamond substrate, the improvement for enhancing diamond nucleation comprising the steps of: a) forming on at least a portion of the substrate a nucleating layer comprising C₇₀ fullerene, and b) contacting the nucleating layer and a carbon-bearing reducing plasma while the substrate is electrically biased at a negative potential relative to said plasma to nucleate diamond at said nucleating layer.

31. An article comprising a substrate and a diamond layer nucleated and grown on at least a portion of said substrate having a nucleating layer provided thereon as a carbon cluster having a geodesic molecular structure.

32. The article of Claim 31 wherein the substrate comprises a non-diamond substrate.

33. An article comprising a substrate and a diamond layer nucleated and grown on a selected region of the substrate having a nucleating layer provided thereon as a C₇₀ fullerene or a portion of its molecular structure.

34. The article of Claim 33 wherein the substrate comprises a non-diamond substrate.

35. The article of Claim 34 wherein the diamond layer comprises a component of an electronic device formed on said substrate.

36. An article comprising a nondiamond substrate and a diamond layer nucleated and grown on a selected region of the substrate having a nucleated layer deposited thereon as C₇₀ fullerene.

37. The article of claim 36 wherein the diamond layer comprises a component of an electronic device formed on said substrate.

38. The method of Claim 1 wherein the nucleating layer comprises C₈₄ fullerene.

39. The method of Claim 1 wherein the nucleating layer comprises buckytubes.

40. The method of Claim 13 wherein the carbon bearing reducing gas comprises a mixture of hydrogen and

CH₄.

41. The method of Claim 17 wherein the nucleating layer comprises C₈₄ fullerene.

42. The method of Claim 17 wherein the nucleating layer comprises buckytubes.

43. The article of Claim 31 wherein the nucleating layer comprise buckytubes.

44. The article of Claim 31 wherein the nucleating layer comprise C₆₀ fullerene.

45. The article of Claim 31 wherein the nucleating layer comprises C₈₄ fullerene.

46. The article of Claim 31 wherein the diamond layer comprises a component of an optical device formed on said substrate.

47. The article of Claim 31 wherein said component of an electronic device is a semiconductor.

48. In a method of forming a diamond layer on at least a portion of a substrate, the improvement for enhancing diamond nucleation comprising the steps of: a) forming on at least a portion of the substrate a nucleating layer comprising a carbon cluster having a geodesic molecular structure. b) ionizing the carbon bearing gas, and c) contacting the nucleating layer with said ionized carbon bearing plasma under conditions effective to nucleate diamond at said nucleating layer.

49. The method according to Claim 48 wherein the nucleating layer comprises C₇₀ fullerene.

50. The method according to claim 48 wherein the nucleating layer comprises C₆₀ fullerene.

51. The method according to Claim 48 wherein the nucleating layer comprises buckytubes.

52. The method according to Claim 48 wherein the carbon bearing gas comprises a mixture of hydrogen and a hydrocarbon.

53. In a method of forming a diamond layer on at least a portion of a substrate, the improvement for enhancing diamond nucleation comprising the steps of: a) forming on at least a portion of the substrate a nucleating layer comprising a carbon cluster having a geodesic molecular structure, b) subjecting a carbon bearing gas to conditions

sufficient to generate its ion, its free radical or excited state thereof, c) contacting the nucleating layer with said modified carbon bearing gas of step (b) under conditions

sufficient to nucleate diamond at said nucleating layer.

54. In a method of forming a diamond layer on at least a portion of a substrate, the improvement for enhancing diamond nucleation comprising the steps of: a) forming on at least a portion of the substrate a nucleating layer comprising a carbon cluster having a geodesic molecular structure, b) subjecting said nucleating layer to conditions sufficient to generate the radical thereof or an excited state thereof, c) contacting the modified nucleating layer of step (b) with a carbon bearing gas under conditions sufficient to nucleate diamond at said nucleating layer.

55. The method according to Claim 53 or 54 wherein the nucleating layer comprises C₇₀ fullerene.

56. The method according to Claim 53 or 54 wherein the nucleating layer comprises C₆₀ fullerene.

57. The method according to Claim 53 or 54 wherein the inert gas is helium or argon.

58. The method according to Claim 53 or 54 wherein the carbon bearing gas comprises a mixture of hydrogen and a hydrocarbon.

59. The method according to Claim 53 wherein step (b) comprises contacting the carbon bearing gas with an inert ion.

60. The method according to Claim 59 wherein the inert ions are ionized helium, ionized argon or ionized.

61. The method according to Claim 53 wherein step (b) comprises contacting the carbon bearing gas with a beam wherein the beam is an ion beam, laser beam or electron beam.

62. The method according to Claim 54 wherein step (b) comprises contacting the nucleating layer with an inert ion.

63. The method according to Claim 62 wherein the inert ions are ionized helium, ionized argon or ionized.

64. The method according to Claim 53 wherein step (b) comprises contacting the carbon bearing gas with a beam wherein the beam is an ion beam, laser beam or electron beam.

65. The method according to Claim 54 wherein step (b) comprises applying an electrical field to said nucleating layer from an external power source connected to said substrate to cause said nucleating layer to be fragmented.

66. The method according to Claim 53 wherein step (b) comprises exciting said carbon bearing gas.