Background of the Invention The present invention relates to the growing of large diamond crystals and to the growing of polycrystalline films of diamond of controlled orientation and crystallite size at higher rates and in a more controlled manner than was previously possible.

It is, of course, well known that diamond crystals can be grown or produced synthetically by closely controlling the requisite chemical and physical conditions. Typically, such processes are carried out under extremely high pressures and temperatures, e.g. about 60,000 atmospheres and 1700°C. These conditions are obviously difficult and expensive to maintain and more recent efforts have been directed at the production of diamond crystals under low pressures, i.e. below atmospheric pressure and at more moderate temperatures.

The use of low pressure techniques requires careful control of other conditions, since below atmospheric pressure diamond is the unstable form of carbon, and graphite is the stable form. Thermodynamically, a stable solid should form preferentially over an unstable solid; however, it has been well-established that diamonds can be grown from energetically activated gases at low pressures in spite of the theoretical thermodynamic instability.

Typical conditions at which such diamonds are grown are a total pressure of about twenty (20) Torr, gas composition of one volume percent methane in hydrogen, and a substrate temperature of about 900°C. Typically, energy is added to the gas by a number of

eans including use of a heated filament, e.g. tungsten, or a microwave discharge. It is generally believed that the energy added to the gas aids growth of the diamond crystal by fragmenting the hydrocarbon molecules, e.g. methane, into a more chemically reactive species such as methyl radicals, and it is also believed to cause the dissociation of the molecular form of hydrogen, H ₂ , to atomic hydrogen, which is also believed to enhance the growth process.

In addition to the obvious difficulties of maintaining the growth control and the reaction conditions, the foregoing methods cannot generally be successfully employed to grow large crystals of diamond, or polycrystalline deposits of a single orientation. Thus, the low pressure methods described above typically yield partially oriented or unoriented polycrystalline diamond films with average crystal sizes generally in the range of one half to about ten microns.

One of the parameters which has been very closely examined in the prior art is the effect of temperature on the growth rate of diamonds at low pressure. Spitsyn et al. (1981) teaches that a maximum growth rate is achieved at a substrate temperature of approximately 950°C and that, as the temperature is then increased, the growth rate decreases until temperatures above 1100°C, the growth rate is essentially zero. Other studies, on the other hand, show that diamond growth can occur at temperatures well above 1100°C, including at least one instance in which a diamond was grown by a thermally-activated chemical vapor deposition on diamond

substrate crystals at 1475°C.

Because graphite is the thermodynamically stable phase of carbon, it was believed that it, and related graphitic materials, should be rigorously excluded from the diamond growth chamber. In fact, it was believed that one of the primary functions of the atomic hydrogen produced in the growth chamber was to remove all traces of graphitic forms of carbon.

In order to promote the nucleation of polycrystalline diamond films, the conventional procedure is to treat the surface with diamond powder prior to the growth process.

The practical utility of the diamond that is deposited depends on its crystal size and orientation. For some applications, e.g. in electronics, large single crystals of diamond are the most useful. For other applications, polycrystalline diamond films containing highly oriented crystals are preferred. For other applications, e.g. optical coatings and tribological coatings, polycrystalline films of very small, unoriented crystals are the best.

Drawings FIGS. 1-3 are schematic drawings of an apparatus suitable for use in achieving a higher mass transfer rate, one of the embodiments of the present invention.

Summary of the Invention It has now been found that the size, orientation, and number of diamond crystals can be controlled by using as a substrate a non-diamond material such as graphite and related aromatic ring compounds, beryllium oxide (BeO) and hexagonal boron nitride (h- BN) . The use of these substrates in the manner described gives rise to improved diamond deposits. The diamond can be further- improved on these and even on conventional substrates such as molybdenum or diamond by effecting an increase in the rate of mass transfer. In general, the process involves the use of apparatus of the type conventionally used heretofore for subatmospheric pressure deposition of diamond films, for example, a heated filament or microwave energy means, a gaseous composition of hydrogen and a suitable gaseous source of carbon such as methane or other hydrocarbon at a pressure of from about 5 Torr to about 100 Torr, and at a substrate temperature of from about 700°C, to about 1450°C.

Generally, at temperatures below about 700°C, the growth rate of the diamond film becomes much slower and the deposit tends to be a smooth polycrystalline film with small, unoriented diamonds, while temperatures above about 1450°C tend to favor the deposit of graphite. Similarly, below about 5 Torr, the rate of deposit is extremely slow and it is nearly impossible to efficiently deposit diamonds while at pressures above about 100 Torr, the formation of graphitic materials rather than diamonds tends to be favored in

hot-filament and microwave reactors.

Also, it has been found that when the gas phase concentration of methane becomes too high, non-diamond forms of solid carbon start to precipitate. In conventional hot-filament diamond deposition reactors this occurs at concentrations of methane greater than about three volume percent in hydrogen gas. The precipitation of the non-diamond, graphitic forms of carbon in this manner limits the growth rates that one can achieve in hot-filament reactors to around one micron per hour for example. It has been found that, when the deposit is made on graphite, the resulting diamond has a preferred orientation with respect to the orientation of the graphite on which it grew.

Furthermore, the number of diamond particles is greatest along the edges of the graphite sheets. This makes it possible to control both the orientation and the number of diamond crystals. For example, to grow many, non-oriented diamond crystals, one uses as a substrate very small flakes of non-oriented graphite flakes. To grow a few, highly oriented diamonds, one uses a single sheet of highly oriented graphite. It is very surprising that graphite (and related compounds) , which heretofore had been avoided, can actually be advantageously used to give diamond films controlled, improved properties.

The use of BeO and h-BN as substrates, which give oriented diamond crystals and films of diamond, is also surprising since neither BeO or h-BN has the same crystal structure as a diamond. It will be even more obvious that, while the growth of superior

diamond deposits on a non-diamond substrate is itself highly unexpected, the deposit of such growths on a graphite substrate is very highly unexpected and clearly contrary to all the teachings heretofore known in the art. Diamond itself is a very attractive substrate, especially for growth of large diamonds. A small diamond crystal can be used as a seed on which a much larger diamond crystal can be grown. Also, the orientation and the type of facets exposed to the growth environment can be controlled to enhance the growth of high quality diamonds. For example, growth on (100) faceted diamond leads to fewer stacking faults and dislocations than growth on (111) faceted diamond. Also, the use of multiply twinned diamond crystals can lead to greatly enhanced growth rates. The proper choice of seed crystal, when used with the moving substrate reactor described herein, leads to much greater growth rates of high quality diamond crystals. Also, the growth on (110) facets has been found to be more rapid than on (100) or (111) facets.

The results of the present experiments would seem to indicate that the maximum in diamond growth rate heretofore observed in the prior art is a maximum in the rate of nucleation of new diamond crystals, and is not a maximum in the inherent growth rate of an existing individual crystal of diamond. In other words, an existing diamond crystal can grow when held at temperatures of 1100°c and above, if supplied with the proper source of carbon and hydrogen containing species from the gas. However, above 1100°C the nucleation of new diamond crystals is suppressed.

It has also been found that very well-oriented crystals of high quality diamond can be grown on graphite or beryllium oxide

(BeO) substrates. It has been further found that appropriate control of the substrate temperature and the supersaturation of carbon, hydrogen, and oxygen in the gas phase will not only suppress the nucleation of new, non-oriented crystals, but also suppress the conversion of BeO to Be, and promote the growth of diamond films and crystals with controlled crystal size and hence with very useful properties. We have further discovered that the growth rate of diamond is strongly increased when the rate of transport of chemical species and of energy to the growing diamond surface is increased. This discovery has led to a new diamond deposition reactor concept with very significant advantages over previous methods.

The Preferred Embodiment

In the preferred method of the present invention, the process is carried out at a temperature of at least about 700°C employing a gas composition of from about 1/2 to about 3% methane and hydrogen. The preferred range of concentration of hydrocarbon in the gas composition can be higher when oxygen-containing compounds such as ethanol or acetone are employed. The preferred composition and pressure range can also be extended by providing means for enhancing the rate of transport of the active species to the substrate. These active species are believed to be, for example, atomic hydrogen and methyl groups. A particular means of enhancing

the transport rates of these and other active species by using a rapidly moving substrate is described hereinafter. In one process of the present invention, the substrate is a diamond seed crystal. In another preferred process of the present invention, the substrate holder is treated with graphite or with a compound containing a plurality of condensed aromatic rings preferably at least about five aromatic rings, such as for example, perylene tetracarboxylic acid dianhydride (PTCDA) .

While PTCDA is one preferred condensed aromatic ring compound (hereinafter referred to as "ring compound(s)••) to achieve enhanced nucleation, it will be obvious that there are a number of similar compounds which can be employed, the critical condition being that the compound must be one which will remain nonvolatile under the anticipated experimental conditions. Thus, there are compounds which might be unsuitable or use where the process is conducted at the upper portion of the temperature range, i.e. between 1300° to about 1400°C, which would still be very useful if employed where the process is conducted at the lower portion of the temperature range, i.e. about 700°C to about 900°C. Also, the effectiveness of a particular material as a nucleating agent for diamond will depend critically on the rate at which treated substrate is heated and whether or not the particular material forms a chemical bond with the substrate being used. Rapid heating will aid in reaching the diamond nucleating conditions before the nucleating agent has had time to vaporize away. If an otherwise volatile nucleating agent forms a strong chemical bond with the surface of the substrate, it

may remain non-volatile and attached to the substrate at high temperatures where it can be effective in promoting nucleation. Compounds containing oxygen may, for example, form Si-O- bonds with a silicon surface. The following examples will serve by way of illustration and not by way of limitation to further describe the process of the present invention and the results which can be achieved by employing it.

Diamond Growth vs. Diamond Nucleation Unless otherwise specifically noted, the examples which follow were conducted employing apparatus of the type described by Angus and Hayman in an article published in Science. Vol. 241, August, 1988 (pgs. 913-921) and more particularly apparatus of the type defined by schematic diagram A on page 914.

Example 1

A molybdenum substrate is held at 950°C and at a distance of approximately 0.5 cm. from a tungsten filament held at 2200°c. Both substrate and filament are placed in a quartz tube so that gas can be passed over them. A flow of 1% CH ₄ in H ₂ at a total pressure of 20 Torr is passed through the tube. Diamond crystals nucleate and grow on the substrate and eventually form a continuous diamond film. The crystallite sizes range from approximately one half to three microns.

Example 2

The same conditions of Example 1 are used except that the temperature of the substrate is reduced to 800°C. A diamond film is formed, but the crystallites have sizes much less than one micron. The surface of the diamond film is much smoother than the sample grown under the conditions of Example 1. At still lower substrate temperatures, the film becomes so smooth, i.e. the particle size is so small, the colored interference fringes are easily observed. The average linear growth rate of the film measured perpendicular to the surface is roughly the same as in Example 1, i.e. approximately one micron per hour.

Example 3

The same conditions of Example 1 are used except that the temperature of the substrate is increased to approximately 1100°C. The nucleation of new crystals is greatly suppressed, but not completely eliminated. It is found, however, that the crystals that do nucleate grow to much larger sizes, for example to more than one hundred microns in linear dimension. The ultimate size of the crystals is limited by how long one keeps them at the growth conditions. Example 4

A small crucible containing diamond powder of nominal size range from zero to one micron was heated to various temperatures while gas mixtures containing hydrogen were passed over it. Temperatures in excess of 1400°C were used. It was found that the

diamond powder in the crucible did not spontaneously transform into graphite even at the highest temperatures. This is a necessary, but not sufficient, condition for achieving growth of diamond.

Example 5 The same apparatus used in Example 4 was used to grow diamond. Pressures ranged from 0.01 to 60 Torr. Gas phase compositions ranged from pure CH ₄ to 1/1 mixtures of CH ₄ and H ₂ . Temperatures were varied from approximately 1100°C to over 1400°C. It was observed that diamond grew at temperatures as high as 1475°C.

Example 6

The same apparatus used in Example 4 was used. The growth rate of diamond was accurately determined by measuring the mass change of the crucible plus the diamond. It was observed that the growth rate of diamond increased in a uniform manner as the temperature was increased. It was furthermore found that the growth rate of diamond obeyed the approximate expression:

R. = R ₁₁₀₀ exp{27,500[l/(T + 273) — 1/1373]} where R, is the diamond growth rate at temperatures above 1100°C, R ₁₁₀₀ is the growth rate at 1100°C, and T is the temperature in °C. This equation fits the data approximately, but is not an exact expression. The equation suggests that one may be able to estimate how increasing the temperature influences the growth rate of diamond. For example, if the temperature is increased from 1100°C to 1300°C, the rate of growth of the diamond mass increases by a

factor of approximately 13. If the temperature is increased from 1100°C to 1400°C, the growth rate increases by over 36 times.

Example 7

A hot-filament reactor was employed in which the substrate temperature could be rapidly changed while keeping all other conditions the same. A flow of H ₂ of 100 seem was passed through a bubbler containing ethanol at room temperature and then into the deposition reactor. The hot filament was maintained at approximately 2000°C. The substrate temperature was increased from 800°C to 1100°C during the first half hour of the run and maintained at 1100°C for six hours. Well faceted diamonds 100 μm in linear dimension and greater were observed.

General Experimental Procedure Unless otherwise noted, the following General Experimental Procedure was employed. A substrate was placed in a molybdenum substrate holder in a hot-filament chemical vapor deposition reactor. A molybdenum shutter was interposed between the substrate holder and the tungsten filaments. A flow of 100 seem of H ₂ at 20 Torr was started and the filament temperature set at 2000°C. The temperature of the substrate holder during this procedure was approximately 500°C. After 15 minutes, a flow of 0.5 seem of CH ₄ was started. After another 10 minutes, the shutter was removed and the distance between the substrate holder and the ilaments was reduced from 22 mm to 8 mm. Throughout the run the substrate

temperature was approximately 830°C. The substrate holder was rotated at 1/3 rpm to insure deposition uniformity. The run continued for seven hours. At the end of the run, the substrate was examined by scanning electron microscopy (SEM) and by Raman spectroscopy. Well-faceted diamond crystals were observed.

Beryllium Oxide Substrate Experiments Example 8

A BeO substrate with the basal plane (0001) exposed was placed in a molybdenum substrate holder in a hot-filament chemical vapor deposition reactor. A molybdenum shutter was interposed between the substrate holder and the tungsten filaments. A flow of 100 seem of H ₂ at 20 Torr was started and the filament temperature set at 2000°C. The temperature of the substrate holder during this procedure was approximately 500°C. After 15 minutes, a flow of 0.5 seem of CH ₄ was started. After another 10 minutes, the shutter was removed and the distance between the substrate holder and the filaments was reduced from 22 mm to 8 mm. Throughout the run the substrate temperature was approximately 830°C. The substrate holder was rotated at 1/3 rpm to insure deposition uniformity. The run continued for seven hours. At the end of the run, the BeO was examined by scanning electron microscopy (SEM) and by Raman spectroscopy. Hexagonally shaped crystals of the same orientation were observed. These crystals were examined by Raman spectroscopy and found to be diamond as evidenced by a strong, narrow Raman line at 1332 cm" ¹ .

One of the diamond samples was examined by cross-sectional transmission electron microscopy. It was confirmed that the orientation of the (111) planes of the diamond were exactly parallel with the (0001) basal plane of the BeO within the accuracy limits of the measurement. It was also confirmed that the <110> direction in the diamond (111) plane was parallel to the <1120> direction in the BeO plane to within an accuracy of 6%.

Example 9

A BeO crystal with an exposed basal plane surface (0001) was mounted in air on a water cooled copper block. The flame from an oxyaeetylene torch was put on the BeO substrate so that the "feather" of the flame was touching the BeO. The flow rates of acetylene and oxygen were 143 and 150 seem respectively. The angle of the flame to the substrate surface was 45°. The estimated substrate temperature was 1250 ± 50°C. The run time was 30 minutes. After the run, the BeO substrate was examined by scanning electron microscopy, atomic force microscopy and by Raman spectroscopy. Many oriented, hexagonally shaped crystals of diamond were found. Some of these crystals were 40 μm in dimension.

Example 10

A sample of BeO was placed in a horizontal tube furnace and a 100 seem flow of molecular hydrogen at 20 Torr with a small partial pressure of water vapor was led into the reactor. A hot tungsten

filament at 2000°C was approximately 0.8 cm from the BeO substrate. Under these operating conditions, the estimated mole fractions of atomic hydrogen and of water vapor are 0.02 and 0.002 respectively. These correspond to partial pressure of:

p _H * 5 x 10~ ⁴ atm and p _{H O} * 5 x 10 ^"5 atm

The run was continued for more than one hour and the BeO sample removed. There was no evidence that the BeO had been reduced to elemental Be.

Example 11

A BeO crystal with an exposed prism plane was placed in a molybdenum substrate holder in a hot-filament chemical vapor deposition reactor. The procedure and experimental conditions were the same as in Example 8. At the end of the run, the sample was examined by scanning electron microscopy, electron microscopy, and electron diffraction. Diamond crystals of the same orientation were found. The diffraction results showed a fixed relationship between the orientation of the diamonds and the BeO.

While unwilling to limit ourselves to any one theory which might explain the unique results achieved employing the method of the present invention, the discussion which follows describes some possible explanations.

Nucleation of New Diamond Crystals The gas phase in contact with the diamond crystal during growth is a complex mixture of many molecules and molecular fragments. The gas composition is dominated by low molecular weight molecules such as methane, acetylene, ethylene, and hydrogen. Also present are unstable radicals and fragments such as atomic hydrogen and methyl radical. The high molecular weight compounds are present in much smaller amounts, but the foregoing results suggest that they may play a major role in determining the nature of the diamond formed.

For example, it is believed that the suppression of nucleation of new diamond crystals may be caused by a greatly decreased concentration of the saturated hydrocarbon ring compounds that can serve as the molecular precursors to new diamond crystals. These precursors are, for example, the "boat-boat" conformers with two parallel twin planes. As will be seen hereinafter, related, higher molecular weight ring compounds can also serve as the diamond precursors.

At the higher temperatures, the concentration of these precursor species is lower because their decomposition into smaller molecules and molecular fragments, e.g., acetylene, methane and methyl radicals, is favored. Therefore, at the higher temperatures one still has the low molecular weight species present that can add to an existing crystal and cause it to grow, but one does not necessarily have sufficient amounts of the high molecular weight

species present to cause the nucleation of new diamond crystals. This can provide greatly increased control over the diamond deposition process, which in turn can lead to useful diamond products, for example, large single crystals of diamond and diamond films of controlled crystallite size.

Growth of Oriented Crystals on BeO The present invention also uses one preferred substrate (BeO) and a particular choice of reaction conditions to achieve growth of diamond crystals of controlled size and orientation. The beryllium oxide (BeO) substrate may act as an atomic template for initiating the growth of oriented diamond crystals. Control of the experimental conditions can then be employed to suppress the spontaneous, secondary nucleation of graphite and of diamond crystals of improper orientation. Both the proper substrate and the proper experimental conditions seem to be required to grow diamond crystals of controlled size and orientation.

One way in which we define the operating conditions is in terms of the chemical potentials of carbon, hydrogen, oxygen, and beryllium in order to make clear the generality of the methods that we propose. Various combinations of temperature, pressure and gas phase compositions can be used to achieve the required range of chemical potentials.

In general, it is required that in order to grow diamond the chemical potential of carbon, μ _c in the gas phase be greater than the chemical potential of diamond. This condition can be written:

χ--Cχa ** Mc.diimαiid

However, since diamond is unstable with respect to graphite, it is also true that c.diunoad ^> c.gitjAite Comparing these two conditions, one sees that when diamond is growing, one also meets the conditions for growing graphite, i.e. c,gιs ^> cgnphhe

However, experimentation has shown that these arguments are correct only for larger size crystals. For the smallest nuclei, for example, containing only ten carbon atoms, under certain conditions graphite nuclei can actually be unstable compared to diamond. In other words

"c-gnpb-te nuclei r"C,diamααd

This means that one can find experimental conditions in which the chemical potential of carbon in the gas is greater than that of diamond, but less than that of graphite nuclei. c.gπφhite nuclei ^ c, gu "* c,diπnond

If the chemical potential of carbon in the gas, μ _c gas, is fixed in this range, then diamond can grow, but graphite nuclei will not form. There are many combinations of temperature, pressure and gas phase composition which can be used to meet this requirement. One common method is the use of atomic hydrogen, which appears to cause the chemical potential of diamond to decrease relative to that of graphite. It is especially important

to keep these general principles in mind when using the present invention.

The present invention permits the growth of larger, more highly oriented diamond crystals than has been possible in the past. Oriented crystals up to 40 microns in linear dimension have been achieved. These crystals appear to be in registry with the BeO substrate. When they undergo lateral growth, parallel to the substrate, they appear to form a single crystal diamond film. Even if the diamond crystals are not in absolutely perfect registry with each other, they will form, at worst, a diamond film with very low angle grain boundaries.

Again, as one possible explanation, it may be suggested that the process is effective because the first nearest neighbor atom positions in BeO are the same as in the diamond-cubic lattice, even though BeO has the wurtzite structure, which is not the same as the diamond-cubic structure. It is only the second nearest neighbor positions that are different in these two structures. Also, the bond distance in BeO is only 7% larger than in diamond. Furthermore, the bonding between diamond and BeO is strong, which may promote the desired lateral two-dimensional growth parallel to the substrate rather than the three-dimensional, unordered growth normally achieved by conventional methods.

Lateral growth of the oriented crystals to achieve large area single crystals apparently can be achieved by using conditions which suppress both the secondary nucleation of unoriented diamond crystals and the reduction of the BeO to Be by atomic hydrogen.

The former can be accomplished by ramping the temperature to higher values (greater than 1100°C for example) and/or control of carbon supersaturation through control of the CH ₄ or C^ concentration. The latter can be accomplished by providing small amounts of oxygen containing compounds in the gas phase.

Pretreatment of the BeO surface with oxygen can also serve to etch away surface damage on the BeO that may have arisen from polishing or other actions prior to deposition. The oxygen, for example in the form of atomic oxygen formed in a low pressure discharge, can etch away the damaged layers leaving an undamaged BeO surface that is better suited for the growth of epitaxially- oriented diamond crystals. The etching can also be achieved using water vapor. This is conveniently done by using a nitrogen gas stream that has been saturated with water vapor by passing through a bubbler.

One can also extend the pretreatment of the BeO by actually etching and reprecipitating a layer of BeO on the surface. This reprecipitated layer can have a more perfect structure than the damaged layers that were etched away. This process can be done using water vapor or other oxygen containing gases in a closed or a flow system.

Graphite Substrates Example 12

A silicon wafer was coated with graphite powder. This was achieved by dispersing the flakes in isopropanol and then spin-

coating the dispersion onto the substrate while rotating at 3000 rpm. X-ray diffraction analysis confirmed that the majority of the graphite flakes lay flat on the silicon substrate surface. In other words, the basal (0001) plane of the graphite was parallel with the (001) plane of the silicon substrate surface. Diamond was then deposited on the graphite coated substrate using the procedure described in Example 8. After a seven hour run, well-faceted diamond crystals were observed on the silicon substrate surface. There was no evidence of any residual graphite left. The diamond crystals were examined by X-ray diffraction and were found to have a preferential orientation with the (111) diamond planes parallel to the original substrate surface. A high resolution transmission electron micrograph was made of one of these small diamond crystals. It was found to be oriented with the (111) planes of the diamond parallel to the (001) silicon surface.

Use of Ring Compounds and Graphite Example 13

Perylene tetracarboxylic acid dianhydride (PTCDA) was dissolved in tetra methyl ammonium hydroxide (TMAH) . A silicon wafer was soaked in this solution for ten hours. The treated silicon wafer was then subjected to the diamond deposition process described in Example 8. Greatly enhanced diamond nucleation densities were achieved. This was demonstrated by counting the number of diamond crystals with size greater than 1 micron. For example, the nucleation density on untreated silicon wafers were

8,500 and 14,500 diamonds per square centimeter, while on the PTCDA treated wafer the density was 83,000 diamonds per square centimeter.

Example 14 The same procedure followed in Example 13 was used except that the spherical compound Cg _π (buckminster-fullerene) was used in place of PTCDA. The C _M was physically spread on the surface of the silicon wafer. No enhancement of nucleation of diamond was observed, i.e. the nucleation density was 14,000 diamonds per square centimeter.

Example 15

Large flakes of highly oriented pyrolytic graphite (HOPG) were placed on a (100) silicon wafer, which was then subjected to the same diamond growing conditions described in Example 8. Greatly enhanced nucleation of diamond was found around the edges of the pyrolytic graphite flakes. The measured density of diamond was 220,000 diamonds (with size greater than 1 micron) per square centimeter. The orientation of the diamonds and the graphite were determined by X-ray diffraction. Many of the diamond crystals were found to be oriented with their (111) planes parallel to the basal plane of graphite.

Example 16

The same procedure of Example 15 was followed except that the

deposition time was increased until all of the pyrolytic graphite had been consumed. There were diamond crystals all over the surface of the silicon water. These crystals had a preferential orientation with respect to the silicon substrate surface.

Example 17

Two (100) pieces of silicon were placed in a hot-filament reactor and subjected to the deposition procedure described in Example 8, but for a time period of only two hours. One of the silicon samples was pretreated by abrading with diamond abrasive powder. The other was treated with graphite powder as described in Example 12. After the two hour deposition run, numerous diamonds were found on the silicon sample treated with graphite; the silicon sample treated with diamond powder had only a few, much smaller diamonds.

Example 18

A single flake of highly oriented pyrolytic graphite was placed on a silicon wafer and subjected to the diamond growing conditions described in Example 8. The orientation of two individual diamonds with respect to the graphite was determined by electron diffraction. The (111) planes of diamond were parallel to the basal planes of the graphite and, in the plane, the [220] direction of diamond was approximately parallel with the [1120] direction of the graphite.

Example 19

A substrate of platinum was used in a hot-filament diamond deposition reactor. Platinum appears to be especially suitable for nucleation of graphitic, non-diamond carbons. Conditions were chosen by increasing the temperature and the methane concentration so that initially a thin layer of polycrystalline graphitic material was first deposited on the surface. Then diamond was nucleated on this layer. It was found that the thin layer of graphite formed in-situ in this manner can also serve as a nucleation promoter for diamond.

Example 19A

PTCDA was vapor deposited over the entire surface of a silicon wafer. A small portion of the deposited PTCDA was subjected to bombardment by high energy helium ions. The ion energy was greater than 1 megavolt. The ion bombardment caused the PTCDA to cross link and to bond strongly to the diamond surface. The wafer was then placed in a hot filament reactor and diamond was deposited using the procedure described in the General Experimental Procedure. After removal from the reactor, it was observed that diamond crystals had only grown on that part of the wafer that had been subjected to the bombardment by helium ions.

Additional Discussion

Seeking diamond nucleation through addition of graphitic materials can be used to grow diamond in controlled geometric

patterns on a surface. For example, a volatile or partially volatile hydrocarbon can be placed on a substrate and then subjected to ion or electron bombardment in a controlled manner. This bombardment will cause the partially volatile material to cross link and to bond more tightly to the substrate, thus making the bombarded part less volatile. When placed in a diamond growing reactor, the more volatile material will leave and the remaining less volatile material will serve as nucleation sites for growing diamond. One of the alternative embodiments of the present invention is to use a patterned graphite or carbon-containing material as the substrate to thereby provide a geometrically patterned diamond film. This pattern can be achieved by ion or electron bombardment of a hydrocarbon or by deposition of a suitable hydrocarbon material through a geometrically patterned mask. Also, the process can be conducted at temperatures below about 1000°C to achieve nucleation of diamond and then use higher temperatures, i.e. higher than about 1000°C for the principal deposition of the diamond crystal. A still further alternate embodiment would be the use of oriented crystals of graphite as the substrate. Such oriented crystals of graphite can be achieved by a number of means. For example, it can be deposited by spin coating of a dispersion of graphite particles in their liquid medium.

As in the use of graphite substrate, a number of alternative techniques can also be employed in conjunction with the method of the present invention. For example, the beryllium oxide substrate

can be deposited, subsequently etched, and regrown one or more times. Such etching can be conducted by a number of techniques well known to those skilled in the art, though the use of an oxygen-containing gas is preferred. As in the case of graphite and condensed aromatic ring substrates, the process of the present invention can be conducted at temperatures below about 1000°C to achieve a nucleation deposit and then temperatures above about 1000°C can be employed for the growing of the crystal itself.

Use of the prism planes of BeO as the substrate surface should favor the growth of the hexagonal form of diamond (lonsdaleite) rather than the normal cubic form of diamond. This is because the atomic scale geometry of the prism faces is similar to that of hexagonal diamond rather than cubic diamond.

One of the alternative embodiments of the present invention is to use a patterned graphite or carbon-containing material on the BeO substrate to thereby provide a geometrically patterned diamond film. This pattern can be achieved by ion or electron bombardment of a hydrocarbon or by deposition of a suitable hydrocarbon material through a geometrically patterned mask. Also, the process can be conducted at temperatures below about 1000°C to achieve nucleation of diamond and then use higher temperatures, i.e. higher than about 1000°C for the principal deposition of the diamond crystal.

Effect of Mass Transport Rates The results of the experiments suggest that the nucleation and growth rates of diamond crystals are strongly influenced by the

rate of mass transfer of species to the surface. Prior work has focused on the effect of chemical reaction rates on limiting the diamond growth rate. Mass transport is enhanced by increasing the relative motion of the fluid phase versus the substrate on which the diamond grows. In all previous diamond reactors, this relative motion was achieved solely by flow of the gas phase over a stationary substrate. However, rapid motion of the substrate, e.g. rotational motion, seems to have important advantages. In addition to increasing the mass transfer rate, the rotation appears to increase the uniformity of diamond deposition as well. This is especially valuable when growing large diamond crystals, e.g. one carat, or when growing large area diamond film, i.e. 10cm ² .

Typical growth rates in hot-filament reactors are only one micron or less per hour. By rapid motion of the substrate, one can substantially increase this growth rate. These growth rates appear to be limited only by the rate at which the active species from the gas phase can move to the substrate. In other words, the growth rates are limited by transport processes in the gas phase and not by inherent limitations of chemical reaction rates on the diamond surface. Ultimately, if the transport of the gas phase active species, e.g., atomic hydrogen and methyl groups, were sufficiently rapid, one would reach a regime in which the growth rate of diamond were limited by the inherent rate of the chemical reactions that cause carbon to be deposited on the diamond surface. However, in the conventional diamond deposition reactors, this limitation has not been reached. Also, in addition to increasing the rate of

transport of active species to the substrate, one can also enhance the transport of carbon containing species awav from the hot- filament. This reduces the tendency of graphitic carbons to deposit on the hot-filament. It is well known that the deposition of graphitic carbons on a hot-filament has a very deleterious effect on the growth of diamond.

Growth rates of 100 or 1000 microns per hour or even higher may be achieved by application of the principles outlined here.

Description of Rotating Substrate Hot Filament Reactor

One simple way of achieving rapid substrate motion is to rapidly rotate the substrate, for example at a speed of from about

500 to about 50,000 rpm. One means of doing this is shown in

Figures 1, 2, and 3. In Figure 1, the overall schematic of a rotating substrate reactor is shown. The reactor is constructed of a stainless steel cross having three arms. The end of one arm is closed by a flange through which a gas inlet and a gas outlet pass. The feedthroughs for providing electrical power to the filaments also go through this flange. The top arm of the cross is fitted with a transparent view port, through which the substrate and the filaments can be observed. The bottom arm of the cross is closed with a flange on which a magnetically coupled mechanical motion feedthrough is attached. This magnetic feedthrough couples the rotational motion of an electric motor to the shaft within the deposition chamber on which the substrate holder and substrate are mounted. Use of a magnetic coupling permits rotational motion to

be generated within the deposition chamber without the necessity of having a shaft passing through into the deposition chamber from the outside. This permits greater control over the purity of the gases within the deposition chamber. The gas inlet shown in Figure 1 can be attached to a manifold to which are connected cylinders of different source gases. These are typically hydrogen and methane, but are not limited to these. Control of the overall gas composition is achieved by mass flow controllers that control the flowrates of the individual gases. The gas outlet is attached to a mechanical vacuum pump that is used to initially evacuate the deposition chamber and to provide means for pressure control during the deposition run. The temperature of the hot filaments can be measured by viewing the filaments through the viewport with an optical pyrometer. Figure 2 shows a detail of the hot-filament assembly. The filaments, which can be rhenium, tungsten, or some other similar material, are mounted on filament posts, which in turn are connected to a boron nitride plate that is supported by stainless steel (304) rods. Figure 3 shows another modification of the rotating substrate reactor. In this modification means are provided for accurately positioning the hot filaments above the substrate in a controlled manner. As described elsewhere in this disclosure, the rate of diamond growth can be enhanced by increasing the rate of transport of active species to the substrate. (One way of doing this is to make the substrate to filament distance as short as possible.)

This is achieved by placing the hot filament assembly on the flange above the substrate and providing for a bellows which can be moved to provide displacement of the filaments in a direction normal to the substrate. The filaments can be moved during growth to maintain an optimal position as the diamond grows. This modification is especially useful when trying to grow large diamonds.

Uses of Rotating Substrate Hot Filament Reactor The rotating substrate hot filament reactor can be used to deposit diamond on any type of solid substrate that can be used in a conventional hot filament reactor. Substrate materials include, but are not limited to, molybdenum, silicon, tungsten carbide, platinum, stainless steel, BeO, L-BN, graphite, and diamond.

Diamond substrates are particularly useful for growing large diamonds, for example, for gemstone applications. BeO substrates are useful for growing large sheets of highly oriented diamond. For some substrate, e.g. stainless steel or tungsten carbide, it may be necessary to provide a silicon interlayer to enhance bonding between the diamond and the substrate. The figures show the filaments above a rotating circular substrate. Other geometries are possible using these general methods. For example, small diamond seed crystals of known orientation can be brazed to a circular molybdenum substrate using a titanium-gold brazing alloy. In another modification, the outside surface of a cylindrical

object can be coated to placing one or more filaments along the axis of the cylinder. For example, the filament can be a helical coiled wire, within which the cylindrical object is placed. This particular geometry is advantageous because the gas velocity near the cylinder wall is more rapid than it is on the circular end of the cylinder. Also cylindrical objects are often used in a variety of applications.

The rotating substrate hot filament reactor can be used to provide a growth environment at the substrate which alternates rapidly with time. This is done by providing gas inlet parts immediately above the rotating surface. For example, if two jets are used which deliver the gas at the center of the substrate, but which are pointed in opposite directions, the substrate will experience two environments each rotation. One of the gas inlets can deliver a concentrated solution of hydrocarbon in hydrogen, while the other can deliver pure hydrogen. This provides an environment in which the chemical potential of carbon experienced by the substrate varies from supersaturated, i.e. μ _Cgω > μ _c,d i _amond ^to one in which it is undersaturated, i.e. μ _CgϊS < μ _c, e _a i _{ύι '} This alternating chemistry can be used to enhance both growth rates and crystalline purity. Obviously, more than two nozzles can be used and different gas combinations can be employed.

The present invention can also provide an alternating environment through the use of two or more filaments placed at different distances from the substrate.

Example 20

A polished, square diamond crystal with sides 1mm in length was placed on a water cooled copper block. The top surface of the crystal was a (100) face. An oxyaeetylene flame was put in contact with the surface at an angle of 45°. The ratio of acetylene to oxygen fed to the torch was 0.96. The "feather" of the flame touched the diamond surface. The total deposition time was 4 hours and 39 minutes. After deposition a "crown" of new diamond was observed along the top edges of the diamond square. This crown was confirmed to be diamond by Raman spectroscopy. The diamond growth rate is greatest at the corners of the original small diamond crystal where the transport of species and energy to the diamond substrate is greatest.

Example 21 A molybdenum substrate was scratched with a tungsten carbide scribe and placed in a tubular diamond deposition reactor. The substrate temperature was 780°C and the hot-filament temperature was 2100°C. The filament to substrate distance was 8 mm. A gas mixture with a C/H atomic ratio of 0.004 was obtained by passing H ₂ through a bubbler containing ethanol and then led into the reactor. Total run time was 7 hours. After the run diamond particles were found to have preferentially nucleated along the lines of the scratches. Further detailed examination showed that the diamond nucleated preferentially on the exposed convex corners of the scratches. This result seems to indicate that the enhanced

nucleation arises because of the increased mass transfer that occurs at the exposed corner. Since diamond was not used to make the scratch, the enhancement clearly is not caused by diamond particles left on the surface.

Example 22

Experimental conditions were essentially similar to Example

12. However, in this example, a molybdenum substrate with a 3 mm diameter hole drilled in it was used as the substrate. The nucleation density of diamond was observed to decrease dramatically with depth of the hole, i.e.

Example 23

The conditions were essentially the same as in Example 12 except a carbon substrate with a 0.3 mm hole was used. Again, the number of nuclei decreased with the depth of the hole, i.e.

Depth. urn Number of Diamonds

0 25

400 10

600 0

The results of Examples 19 and 20 suggest that the rate of nucleation of diamond may decrease as the diffusion path gets

longer, i.e. as the mass transfer rate gets smaller.

Example 24

Diamond deposition was done in a specially modified hot- filament reactor as shown in the drawings. The molybdenum substrate was rotated at 3000 revolutions per minute during the entire deposition process. The gas flow rates were 200 standard cubic centimeters per minute of hydrogen and 10 standard cubic centimeters per minute of methane. The total pressure was 50 Torr. The power to the dual filaments was 420 watts and their temperature was approximately 2100°C. The run continued for 10 hours. After the run was completed, the substrate was removed from the reactor and examined in the scanning electron microscope. Under these conditions, without rotation, no diamond or diamond of very poor quality is observed. However, in this case, diamond crystals were observed. The crystals showed primarily cubic (100) faceting and were from 25 to 35 microns in size. The crystals were tested by Raman spectroscopy and were found to have a strong, narrow Raman signal at 1332 wavenumbers, which is typical for high quality diamond.

Example 25

A run was conducted using the same conditions and the same reactor as in Example 24 except the pressure was 100 Torr. Under these conditions, without rotation, no diamond is observed. However, in this case, diamond crystals were observed. They were

confirmed as diamond by scanning electron microscopy and by Raman spectroscopy.

Example 26

A run was conducted using the same conditions and the same reactor as in Example 24 except no rotation was performed. After several hours, a graphitic deposit built up both on the hot- filament and on the substrate. No diamond was formed on the substrate.

Example 27 A feed gas consisting of 7% methane and the remainder hydrogen was used. The reactor pressure was maintained at 100 Torr. The filament temperature was approximately 200°C hotter than in the previous examples. The run time was 10 hours. The substrate was an untreated molybdenum disk, i.e. no pretreatment to enhance nucleation was performed. After the run, a complete sheet of diamond crystals covered the surface. These experimental conditions will normally not produce diamond.

Example 28

A run was conducted using the same conditions and the same reactor as in Example 24 except that the gas composition was 10% methane in hydrogen and the total pressure was 100 Torr. Diamond crystals were observed at the completion of the run.

Example 29

A run was conducted using the same conditions as Example 24. Diamonds were found not only on the top circular surface of the substrate, but also on the side walls of the substrate.

The rapid motion of the gas in the rotating substrate hot filament reactor permits the use of higher hydrocarbon, concentrations in the source gases than can normally be used to grow diamond. The higher concentrations of hydrocarbons lead to higher diamond growth rates. The reasons that higher hydrocarbon concentrations can be used are not entirely clear. However, one possible reason is that the enhanced gas velocities prevent an accumulation of carbon containing molecular fragments from building up around the hot filament. This, in turn, prevents graphite from depositing on the hot filament. It is known that graphitic deposits on the hot filament poison the capacity of the hot filament to produce atomic hydrogen, which is believed to be necessary for diamond growth.

The maximum hydrocarbon concentrations that can be used depend not only on the motion of the substrate, but also on the presence of oxygen containing materials in the gas phase. For example, the presence of oxygen permits one to significantly increase the concentration of hydrocarbon without getting deleterious deposition of graphitic deposits. The use of substrate motion further enhances this effect.

Also, the beneficial effects of rotation increase as the speed of rotation is increased. Still higher hydrocarbon concentrations can be used and further growth rate enhancements and other benefits become greater at higher rotational speeds. The maximum usable rotational speed may be limited by mechanical considerations such as seals and bearings.

The motion of the substrate causes the gases in the reactor to move much more rapidly than they would if there were no substrate motion. This increase in motion of the gas phase is what is believed to cause the increase in transport rates of active species to the surface and, hence the increase in growth rates. The influence of rotational velocity on the transport rates to the surface can be estimated by computation of the Peclet number, which is a dimensionless quantity defined by: Pe = vL/D, where v is the gas velocity, L is a characteristic dimension of the reactor system, and D is the diffusion coefficient of the particular species under consideration. The value of the Peclet number is a simple estimate of the magnitude of the transport by convection to the transport by diffusion. . Hot filament reactors typically operate at very low values of Peclet numbers, i.e. Pe « 1, which means that these reactors rely only on molecular diffusion to cause the active species to move from the hot-filament to the substrate. Molecular diffusion is typically a slow process and consequently the growth rates that can be achieved are slow. Rapid motion of the substrate, e.g. rapid rotation, can cause the Peclet number to increase by 100 to 10,000 times, depending upon the particular

design of the reactor. In fact, one can achieve conditions where Pe > l, which means that convection is actually contributing more than diffusion to the transport. This can greatly enhance the growth rates that can be achieved.

Again, there are a number of alternative alternate embodiments which can be employed in conjunction of the process of the present invention. For example, multiple nozzles can be used to inject gases of different compositions onto the rotating substrate thereby providing a rapidly alternating chemical environment adjacent to the substrate. Also, two nozzles can be employed; one of which supplies a gas supersaturated in carbon, while the other provides a gas undersaturated in carbon, thereby providing alternating periods of diamond growth and etching.

One of the alternative embodiments of the present invention is to use a patterned graphite or carbon-containing material as the substrate to thereby provide a geometrically patterned diamond film. This pattern can be achieved by ion or electron bombardment of a hydrocarbon or by deposition of a suitable hydrocarbon material through a geometrically patterned mask. Also, the process can be conducted at temperatures below about 1000°C to achieve nucleation of diamond and then use higher temperatures, i.e. higher than about 1000°C for the principal deposition of the diamond crystal. A still further alternate embodiment would be the use of oriented crystals of graphite as the substrate. Such oriented crystals of graphite can be achieved by a number of means. For

example, it can be deposited by spin coating of a dispersion o graphite particles in their liquid medium.

The patents, patent applications, and other publications se forth in the specification are intended to be incorporated b reference herein.

It is apparent that there has been provided in accordance wit this invention a process and apparatus for producing improve diamond deposits which fully satisfies the objects, means, an advantages set forth hereinbefore. While the invention has bee described in combination with the specific embodiments thereof, i is obvious that many alternatives, modifications, and variation will be apparent to those skilled in the art in light of th foregoing description. Accordingly, it is intended to embrace al such alternatives, modifications, and variations as fall within th spirit and scope of the appended claims.