NASA Technical Reports Server (NTRS) 20110014688: Synthesizing Diamond from Liquid Feedstock

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by  NASA Technical Reports Server (NTRS)

Synthesizing Diamond From Liquid Feedstock
Precise proportioning of feedstock gases is not necessary.
Goddard Space Flight Center, Greenbelt, Maryland
A relatively economical method of A typical apparatus used in this becomes heated by the filament to a de-
chemical vapor deposition (CVD) has method includes a reservoir containing position temperature in the approxi-
been developed for synthesizing dia- the feedstock liquid and a partially mate range of 800 to 1,000 °C.
mond crystals and films. Unlike prior evacuated stainless-steel reaction cham- The composition of the feedstock
CVD methods for synthesizing diamond, ber. The reservoir is connected to the must be chosen in conjunction with
this method does not require precisely chamber via tubing and a needle valve other operational param-eters to ob-
proportioned flows of tain a high-quality dia-
compressed gas feed- mond deposit. When the
stocks or the use of elec- feedstock comprises
trical discharges to de- methanol alone, no dia-
compose the feedstocks mond is deposited on the
to obtain free radicals substrate and there is too
needed for deposition much oxidation of the
chemical reactions. In- hot filament and conse-
stead, the feedstocks used quent gradual reduction
in this method are mix- in the diameter of the fil-
tures of common organic ament. When the liquid
liquids that can be pre- contains too much
pared in advance, and de- ethanol, isopropanol, or
composition of feedstock acetone, the filament be-
vapors is effected simply comes coated with
by heating. graphite and thus
The feedstock used in swollen. When the sub-
this method is a solution strate is placed too close
comprising between 90 to the filament, the con-
and 99 weight percent of centration of the etchant
methanol and the bal- radicals is too high, pre-
ance of one or more This Optical Micrographat a magnification of about 500 shows diamond grains in a venting net deposition. In
other oxyhydrocarbons layer ≈10 µm thick on a silicon substrate. The diamond was deposited using a feed- experiments, it was found
that could include ethanol, stock solution of about 4 weight percent ethanol in methanol, a vapor pressure of that choice of an opti-
40 torr (≈5 kPa), a 1.5-mm-diameter tungsten filament wound in a spiral of ≈2.5 cm
isopropanol, and/or ace- diameter and heated by a current of 110 A, and other deposition conditions as de- mum combination of
tone. This mixture of scribed above. composition of the feed-
compounds is chosen so stock, filament tempera-
that dissociation of molecules results in or other suitable flow controller. When ture, filament-to-substrate distance,
the desired proportions of carbon-con- the liquid enters the low-pressure envi- and vapor pressure results in the depo-
taining radicals (principally, CH3) and ronment inside the chamber, it evapo- sition of high-quality diamond (see fig-
of OH, H, and O radicals. Undesirably, rates to form a vapor mixture of the ure) on the substrate. Moreover, it was
the CVD temperature and pressure con- same chemical composition. In addi- found that if the optimum vapor pres-
ditions thermodynamically favor the tion to the inlet for the feedstock liq- sure is established in the chamber be-
growth of graphite over the growth of di- uid, the chamber is fitted with an outlet fore heating the filament, then the fila-
amond. The H radicals are desirable be- connected to a vacuum pump (not ment becomes coated with a thin layer
cause they help to stabilize the growing shown) through a throttle valve (also of carbon that prevents erosion of the
surface of diamond by shifting the ther- not shown) that is automatically con- filament by the etchant radicals during
modynamic balance toward favoring the trolled to keep the pressure at or near the deposition process.
growth of diamond. The OH and O rad- the required value throughout the de- This work was done by Yonhua Tzeng of
icals are desirable because they prefer- position process. Auburn University for Goddard Space
entially etch graphite and other non-dia- Inside the chamber, a spiral filament Flight Center.
mond carbon, thereby helping to ensure made of tungsten, tantalum, graphite, or In accordance with Public Law 96-517,
the net deposition of pure diamond. other high-melting-temperature material the contractor has elected to retain title to this
The non-methanol compounds are in- is electrically heated to a temperature invention. Inquiries concerning rights for its
cluded in the solution because (1) >2,000 °C — high enough to cause dis- commercial use should be addressed to:
methanol contains equal numbers of C sociation of vapor molecules into the Auburn University
and O atoms; (2) an excess of C over O aforementioned radicals. A deposition Office of Technology Transfer
is needed to obtain net deposition of di- substrate — typically, a diamond-pol- 309 Samford Hall
amond; and (3) the non-methanol mol- ished silicon wafer about 2.5 cm square Auburn University, AL 36849
ecules contain multiple carbon atoms — is positioned about 2 cm away from Refer to GSC-14754-1, volume and num-
for each oxygen atom and thus supply the filament. The exact location of the ber of this NASA Tech Briefsissue, and the
the needed excess carbon. substrate is chosen so that the substrate page number.
18 NASA Tech Briefs, January 2005