Method of synthetic diamond ablation with an oxygen plasma and synthetic diamonds etched accordingly

A method for ablating a synthetic diamond having a pitted surface includes applying a colloidal graphite to the surface of the diamond and subjecting it to an oxygen plasma so that preferably approximately 50 microns are removed from the surface of the synthetic diamond. The resulting surface of the diamond is virtually pit free. Preferably, the diamond is then mechanically lapped for finishing.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the etching of synthetic diamond in an oxygen 
plasma. More particularly, the invention relates to methods of coating a 
synthetic diamond prior to plasma etching such that the density and depth 
of surface pits in the diamond are reduced subsequent to plasma etching 
and to finished diamonds prepared in this manner. 
2. State of the Art 
A diamond is an allotrope of carbon exhibiting a crystallographic network 
of exclusively covalently bonded, aliphatic sp.sup.3 hybridized carbon 
atoms arranged tetrahedrally with a uniform distance of 1.545 .ANG. 
between atoms. Due to this structure, diamonds are extremely hard and have 
a thermal conductivity approximately four times that of copper while being 
an electrical insulator. Although diamonds are most popularly known for 
their gemstone qualities, their hardness, thermal and electrical 
properties make them very useful for many industrial applications. 
Naturally occurring diamonds are believed to be the result of pure carbon 
having been subjected to tremendous pressure and heat deep within the 
earth. Synthetic, or man-made, diamonds became possible in 1955, when the 
General Electric Company used laboratory equipment to subject graphite to 
great pressure and heat. Today, diamonds can be grown as an equilibrium 
phase at high pressures or under metastable conditions at low pressures. 
One of the methods developed in recent years for producing diamonds is 
known as chemical vapor deposition (CVD). CVD methods use a mixture of 
hydrogen and a gaseous carbon compound such as methane which is activated 
and contacted with a substrate to produce a diamond film or wafer on the 
substrate. The hydrogen gas is dissociated into atomic hydrogen and then 
reacted with the carbon compound to form condensable carbon radicals 
including elemental carbon. The carbon radicals are then deposited on a 
substrate to form a diamond film. 
One of the persistent undesired features of diamond wafers manufactured by 
CVD is that of large surface irregularities (pits). For most applications, 
it is necessary to smooth or polish (ablate) the surface of the diamond 
film to obtain the proper thickness tolerance and/or surface finish. Due 
to the extreme hardness of diamond, specialized tools are required for 
accurately machining the surface of the diamond film to the desired 
finish. Typical methods for mechanical diamond finishing involve abrading 
the diamond film with a diamond grit slurry on a lapping machine. These 
methods tend to be costly and time consuming, often entailing ablating 
rates of about 0.1 .mu.m/hr to 0.1 .mu.m/min and requiring up to several 
months to finish a four inch diameter diamond wafer. 
It is also known in the art to use a laser or other high energy beam to 
ablate the surface of a diamond film in order to achieve a desired finish. 
The use of lasers and similar high energy beams is in very early stages of 
development. While these methods promise to be less time-consuming and 
more accurate, they can often generate defects in the diamond. In general, 
laser ablation is accomplished by irradiating the surface of the diamond 
with a laser beam at an angle, such that the convex irregularities on the 
surface are exposed to a higher laser power density than the planar areas, 
and the concave irregularities are shielded by the convex ones. The 
surface of the diamond undergoes several passes under the laser to evenly 
smooth the surface and the laser typically removes from 10 to 40 microns 
of diamond per pass under the laser. The result is that a maximum 
convexity height (Rmax) of 50 .mu.m on the surface of a diamond can be 
reduced to 3 .mu.m within a relatively short period of time. It is 
difficult to obtain any particular level of surface smoothness, however, 
unless the surface of the diamond film is ablated in very small increments 
and separately measured after each ablation. Moreover, while a significant 
depth of diamond is removed during each pass under the laser, a typical 
100 mm wafer must be subjected to many passes before its entire surface is 
ablated. 
Recently, diamond films and wafers have been used in microelectronic 
applications as heat sinks or substrates for semiconductor devices. In 
these applications, it is often necessary to etch the surface of the 
diamond. A current technique for etching a diamond wafer is known as 
oxygen plasma etching with pattern masking. In oxygen plasma etching, a 
diamond wafer is lithographically patterned with photoresist or with 
sputtered Ti--Pt--Au. The masked wafer is placed in a reactor with oxygen 
under pressure of 1 to 30 mtorr. Microwave powers of 200 to 700 watts and 
DC biases of -50 to -300 volts are induced in the reactor by applying 
13.56 MHz (an FCC allocated frequency) RF power to a cathode. Plasma 
etching can achieve an etch rate of 5 to 10 microns/hr which is 
substantially faster than other methods. The influence of various 
parameters on the etch rate of the diamond wafer is discussed by Pearton 
et al. in "ECR Plasma Etching of Chemically Vapor Deposited Diamond Thin 
Films", Electronics Letters, 23 Apr. 1992, Vol. 28., No. 9, pp. 822-824. 
The authors conclude that Ti--Pt--Au is more resistant to etching than 
photoresist and has other advantages in that it performs a self-aligned 
contact to the diamond. In any case, however, the plasma etching is highly 
anisotropic at low pressure and the surface morphology of the diamond is 
substantially the same after etching as it was prior to etching. That is, 
pits are not removed. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a new method for 
ablating a diamond wafer which exploits the rapid etching rate of plasma 
etching. 
It is also an object of the invention to provide a method of ablating a 
diamond wafer using plasma etching which overcomes the inherent 
anisotropic nature of plasma etching. 
In accord with these objects which will be discussed in detail below, the 
method of the present invention includes applying colloidal graphite to 
the surface of a diamond wafer and then subjecting the wafer to an oxygen 
plasma. Preferably, the oxygen plasma is permitted to remove approximately 
50 microns of diamond so that the pits are removed (or more accurately, 
the diamond between the pits is removed down to the bottom of the pits). 
Also, the wafer is then preferably mechanically lapped. The resulting 
surface of the wafer is virtually pit free. 
Additional objects and advantages of the invention will become apparent to 
those skilled in the art upon reference to the detailed description taken 
in conjunction with the provided figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning now to FIG. 1, the method of the invention is seen. At 10, a CVD 
diamond sample is obtained. Colloidal graphite (graphite particles 
preferably having a mean diameter of approximately 1 micron suspended in a 
solvent) is then applied to the surface of the diamond sample at 12. The 
surface bearing the colloidal graphite is subjected to oxygen plasma 
etching at 14. Optionally, after applying the colloidal graphite at 12, 
and prior to plasma etching at 14, the surface bearing the colloidal 
graphite is cleaned at 13. In addition, after plasma etching at 14, the 
surface upon which the colloidal graphite was applied is optionally lapped 
at 15. The described method of the invention will be better understood by 
reference to the following example. 
A diamond wafer formed by a CVD process, and having a diameter of 
approximately 100 mm was subjected to plasma etching on the substrate side 
for approximately nine hours. The average removal of diamond from the 
wafer was approximately 41.0 microns with a standard deviation of 
approximately 6.5 microns. Commercial grade colloidal graphite (graphite 
suspended in a solution of water and alcohol) was then applied to a 15 mm 
wide strip of both sides of the wafer. The wafer was then subjected to a 
second etching procedure on its deposition side where the average removal 
of diamond from the wafer was approximately 45.6 microns with a standard 
deviation of approximately 16.0 microns. The wafer was then subjected to a 
third etching to remove approximately 50 microns from the deposition side. 
All of the etchings were performed in a reactor in the presence of oxygen, 
argon, and SF.sub.6 under a pressure of 4 mtorr. Input power to the 
reactor was held constant at 600 W and the RF bias was held to a constant 
maximum -118 V. The flow rates of oxygen, argon and SF.sub.6 were fixed at 
28, 6, and 2 sccm, respectively. The input power to the reactor may be 
varied from about 500 to 1500 W and the RF bias may be varied from about 
-100 to -300 V. While oxygen is generally considered to be necessary to 
the etching process, argon and SF.sub.6 may not be necessary to achieve 
the desired results. Argon is used to help ignite the ECR plasma, but the 
process will probably work without argon. SF.sub.6 prevents the formation 
of black film which may be acceptable in some applications. The flow rates 
of oxygen, argon and SF.sub.6 may be varied from 20-40, 0-20, and 1-4 
sccm, respectively. The pressure of the reactor may be varied from 2-10 
mtorr. 
After the above described etching treatments were concluded, the wafer was 
mounted in a lapping machine. The deposition side of the diamond wafer was 
mechanically lapped for approximately sixty-three hours. Visual inspection 
of the wafer after lapping revealed an absence of pits on the portion of 
the wafer which had been coated with colloidal graphite. The wafer was 
lapped an additional twenty-one hours and micrographs were taken of the 
lapped surface. 
FIG. 2 is a micrograph (65.times. magnification) of a portion 20 of the 
deposition side of the diamond wafer which was not coated with colloidal 
graphite. The large dark spots 25, 30 in FIG. 2 represent pits in the 
surface of the diamond. These pits are on the order of 50 microns deep. 
FIG. 3 is a micrograph (65.times. magnification) of a portion 40 of the 
deposition side of the diamond wafer which was coated with colloidal 
graphite. The absence of large dark spots in FIG. 3 represents a pit-free 
surface of the diamond wafer. 
The above procedures were repeated using other diamond samples including a 
2 cm square and another 100 mm disk. Similar results were obtained with 
each sample. From the foregoing, it is believed that the colloidal 
graphite coating interacts synergistically with the plasma etching process 
to reduce the depth of pits on the etched diamond surface. While the 
results of the procedure are most apparent after the surface is further 
ablated by lapping, it is believed that significant removal of pits is 
obtained prior to lapping, as lapping does not remove pits, but simply 
finishes the surface. In addition, it is noted that ablation methods other 
than lapping (e.g., grinding or laser ablation) may produce similar 
results in a diamond sample which has first been subjected to coating with 
colloidal graphite and then plasma etched. 
There have been described and illustrated herein a method for ablating a 
synthetic diamond, and the synthetic diamonds which result from the 
method. While particular embodiments of the invention have been described, 
it is not intended that the invention be limited thereto, as it is 
intended that the invention be as broad in scope as the art will allow and 
that the specification be read likewise. Thus, while particular parameters 
have been disclosed with regard to the plasma etching reactor, it will be 
appreciated that other parameters could be utilized. Also, while 100 mm 
disk and 2 cm square diamonds have been shown, it will be recognized that 
other configurations of diamonds could have their surfaces finished 
according to the method with similar results obtained. Moreover, while the 
colloidal graphite used contained particles having a mean diameter of one 
micron, it will be understood that other particle sizes may achieve the 
same or similar results. It will therefore be appreciated by those skilled 
in the art that yet other modifications could be made to the provided 
invention without deviating from its spirit and scope as so claimed.