Transparent diamond films and method for making

A method is provided for making substantially transparent polycrystalline diamond film having a thickness greater than 50 microns which can be used in glazing applications and as a heat sink in microelectric applications. A mixture of hydrogen and methane is conveyed into a heated filament reacting zone which is adjacent to an appropriate substrate, such as a molybdenum substrate to produce non-adherent polycrystalline substantially transparent diamond film.

BACKGROUND OF THE INVENTION 
The present invention relates to vapor deposited transparent 
polycrystalline diamond films. More particularly, the present invention 
relates the method of introducing a particular mixture of hydrogen and 
methane into a heated reaction zone adjacent to a substrate such as 
molybdenum, to effect polycrystalline diamond film deposition. 
As taught by Spear, Diamond-Ceramic Coating Of The Future, Journal of 
American Ceramics Society, 72[2]171-91 (1989), the growth of 
single-crystal films of diamond is critical to many electronic and optical 
applications, but it is a feat that has not been achieved except for 
homoepitaxial growth on diamond substrates. There is reported by Peter K. 
Backmann, et al in the May 15, 1989, edition of Chemical and Engineering 
News, on page 38, that vapor deposited diamond heat sinks have been 
developed using plasma jet deposition to produce polycrystalline material 
up to 4.times.6.times.1 millimeter. 
In Japanese patent 85,141,697, it is reported that free-standing diamond 
films have been found useful as diaphragms for speakers. S. Kawachi et al, 
Japanese patent 85(60)-127,292, reports that 10 micron diamond films have 
been deposited on a graphite substrate. K. Fujii, et al, Japanese patent 
85(60)-186,500 teaches that a 6.5 micron thick transparent film can be 
produced on a substrate using a methane- hydrogen mixture. 
Although various procedures have been developed to make vapor deposited 
polycrystalline diamond film, it would be desirable to provide glazing 
materials in the form of free-standing polycrystalline transparent diamond 
films having thicknesses of from 50 to 5000 microns with lateral 
dimensions exceeding 10 millimeters. 
SUMMARY OF THE INVENTION 
The present invention is based on the discovery that vapor deposited 
transparent polycrystalline diamond film can be made at thicknesses 
greater than 50 microns by passing a particular hydrogen-methane mixture 
through a filament heated reaction zone adjacent to a suitable substrate, 
such as a molybdenum substrate, where the hydrogen-methane mixture 
introduced into the reaction zone has from about 1.5 to about 2 volume 
percent of methane, based on the total volume of hydrogen and methane. 
Surprisingly, a transparent non-adherent polycrystalline diamond film 
having an optical absorbance of 2.1 cm.sup.-1 to 32 cm.sup.-1 can be 
formed at a growth rate of about 0.4 to 1.0 microns per hours. Thicknesses 
of at least 500 microns, and as high as 5000 microns or more, can be made 
having lateral dimensions exceeding 25 centimeters. 
STATEMENT OF THE INVENTION 
There is provided by the present invention, a continuous free-standing, 
substantially transparent, polycrystalline diamond film having a thickness 
of at least 50 microns comprising (A) substantially vertical columnar 
diamond crystals having an average diameter of from about 20 to about 200 
microns and a &lt;110&gt; orientation perpendicular to the base and up to 10,000 
parts per million of chemically combined hydrogen which is sufficient to 
substantially saturate dangling carbon bonds at diamond crystal grain 
boundaries, carbon dislocations, and carbon vacancies and (B) diamond 
crystal grain boundaries separating the columnar diamond crystals of (A) 
where the diamond crystal grain boundaries have a 70.degree.-90.degree. 
orientation to the diamond crystal base. 
In another aspect of the present invention, there is provided a method of 
growing a non-adherent substantially transparent polycrystalline diamond 
film on a substrate which comprises, passing a hydrogen-methane mixture 
through a heated reaction zone at a temperature of about 600.degree. to 
about 1000.degree. C. and at a pressure from about 3 to about 24 torr 
which is sufficient to generate active carbon-hydrogen species in the 
heated reaction zone maintained at a distance of from about 0.3 to about 1 
centimeter from the surface of the substrate, where the hydrogen-methane 
mixture introduced into the heated reaction zone has from 1.5 to about 2 
volume percent of methane, based on the total volume of hydrogen and 
methane.

More particularly, there is shown a quartz bell jar at 10 which can be 
20"-30" tall and about 4"-6" wide having a metal collar at its base at 11 
and a gas inlet at the top at 12. The metal collar portion rests on a 
rubber seal at 13 which can be Neoprene rubber. The rubber seal is 
supported by a metal base, such as steel base structure at 14 which has a 
vacuum opening at 15. 
Inside the quartz bell jar there is shown a supporting stand at 16 for an 
extension at 17 for holding several substrate structures, such as 
molybdenum at 18 and 19 and a filament at 20. The filament is secured by a 
screw at 21 to a metal plug at 22 which passes through a quartz insulating 
collar at 23 which is supported by an extension at 24. Electrical contacts 
are shown from the plug at 25 to a stud at 26 which is insulated from the 
metal base at 27. 
A side view of the polycrystalline diamond film in cross section, and a 
detail at 3A further illustrates the substantially transparent columns of 
diamond crystals having a &lt;110&gt; orientation perpendicular to the base. 
Grain boundaries between adjacent diamond crystals having hydrogen atoms 
saturating dangling carbon bonds are shown at 40 and in detail at 41, 
where at least 50% of the carbon atoms are believed to be tetrahedrally 
bonded based on Raman spectroscopy, infrared and X-ray analysis. 
A detailed discussion of Miller Indices describing crystal planes of atoms 
differentiating between &lt;010&gt;, &lt;110&gt; and &lt;111&gt; orientation is shown on 
pages 65-69 in Elements of Material Science, Second Edition, 1964, by 
Lawrence H. VanVlack of Addison-Wisley Publishing Company, Reading, Mass. 
which is incorporated herein by reference. 
A detailed discussion on chemical bonding and structure discussing the 
hybridization theory and molecular geometry with respect to tetrahedral 
bonding of carbon atoms with hydrogen is shown by Ernest Griswold, 
Chemical Bonding and Structure, pages 55-102, 1968, Raytheon Education 
Company, which is incorporated herein by reference. 
The following shows that dissociation of hydrogen and methane on a heated 
tungsten filament in accordance with the practice of the method of the 
present invention. 
EQU CH.sub.4 (g)+.quadrature.=CH.sub.4 (ad) (1) 
EQU CH.sub.4 (ad)=CH.sub.3 (ad)+H(ad) (2) 
EQU CH.sub.3 (ad)=CH.sub.2 (ad)+H(ad) (3) 
EQU CH.sub.2 (ad)=CH(ad)+H(ad) (4) 
EQU CH(ad)=C(ad)+H(ad) (5) 
EQU C(ad)=C(g)+.quadrature. (6) 
EQU CH(ad)=CH(g)+.quadrature. (7) 
EQU CH.sub.2 (ad)=CH2(g)+.quadrature. (8) 
EQU CH.sub.3 (ad)=CH.sub.3 (g)+.quadrature. (9) 
EQU C(ad)=.quadrature.+C(in filament) (10) 
EQU H.sub.2 (g)+2.quadrature.=2H(ad) (11) 
EQU H(ad)=H(g)+.quadrature. (12) 
.quadrature.=vacant surface site 
(g)=gaseous species 
(ad)=species absorbed on surface. 
The above mechanism is one possible explanation as to how the transparent 
diamond film grows on the substrate. 
As described by Ch. Wild et al. in the First Proceedings For International 
ECS Symposium on Diamond and Diamond-Like Films, Los Angeles, May 7-12, 
1989 for "Optical and Structural Characterization of CVD Diamond", which 
is incorporated herein by reference, infrared and Raman spectroscopy as 
well as X-ray diffraction have been used to investigate polycrystalline 
diamond films prepared by the method of the present invention. The 
absorption spectrum of a 400 micron thick free-standing diamond wafer 
established that the film had a hydrogen concentration of about 5000 part 
per million. Raman spectroscopy was used to establish that the observed 
polycrystalline film was significantly different from graphite, since it 
contained a significant level of tetrahydral carbon atoms. X-ray 
diffraction measurements revealed that the polycrystalline film made in 
accordance with the practice of the present invention had a preferential 
alignment of the &lt;110&gt; planes perpendicular to the growth direction and 
indicated that the diamond crystal grain boundaries had a 
70.degree.-90.degree. orientation to the base. 
The polycrystalline diamond films made in accordance with the practice of 
the present invention can be used in a variety of glazing applications as 
well as heat sinks or semiconductors. 
In order that those skilled in the art will be better able to practice the 
present invention, the following example is given by way of illustration 
and not by way of limitation. 
EXAMPLE 
A mixture of 1.75 volume % of methane and 98.25 volume % of hydrogen, 
measured under atmospheric conditions, was introduced into a reaction 
vessel as shown by FIG. 1. A gas flow rate of about 4000 cubic centimeters 
per minute was maintained. There was used two 11/4".times.1/4".times.9" 
molybdenum substrates and a 9 1/2#218 tungsten filament having a diameter 
of 0.030". The tungsten filament was maintained at a temperature between 
about 2020.degree. to 2040.degree. C. A separation of about 7-8 
millimeters was maintained between the filament and the molybdenum 
substrate during the deposition which lasted approximately 30 days. The 
substrate temperature was estimated at about 800.degree. C. during the 
deposition period. 
At the termination of the 30 day deposition period, the apparatus was 
allowed to cool to room temperature. Transparent polycrystalline diamond 
films having thicknesses of about 500 microns and lateral dimensions 
equivalent to the substrates separated from the substrate during the 
cooling period. 
The diamond films were found to be of good crystalline quality as shown by 
Raman spectra having an intense peak at 1332 cm.sup.-1. The diamond films 
were also found to have the characteristic two phonon adsorption of 
material diamond in the range of 1600-2650 cm.sup.-1 by infrared 
spectroscopy. 
Although the above example is directed to only a few of the very many 
variables which can be used in the practice of the method of the present 
invention to make the polycrystalline diamond films, it should be 
understood that a much broader variety of conditions, apparatus 
arrangements and materials can be used as set forth in the description 
preceding this example.