A process for the fabrication of a polycrystalline diamond membrane involves chemical vapor deposition (CVD) of diamond onto a silicon substrate, followed by conventional photolithography and subsequent reactive-ion etching to remove part of the substrate (see figure). This process is an improvement over an older process in which the substrate is etched in a hot KOH solution. This process can be used to fabricate diamond polycrystalline membranes as parts of microelectromechanical sensors.
The starting substrate is a mirror-smooth (100)-oriented single-crystal silicon wafer with n or p doping to a resistivity <20 Ω·cm. To increase the density of nucleation sites for diamond and thereby make it possible to obtain a pinhole-free diamond deposit, the front (top in the figure) surface of the substrate is scratched by use of diamond paste. A polycrystalline diamond film is grown on the scratched surface by CVD from a flowing mixture or methane and hydrogen, typically at a total pressure of 45 torr (6 kPa) and a substrate temperature of 950 °C.
After deposition of diamond to the required thickness, aluminum is deposited on the back (bottom in the figure) surface of the substrate by electron-beam evaporation. The aluminum film is patterned photolithographically, then etched by a commercial solution containing phosphoric and acetic acids, thereby forming a mask to define the areas to be protected from, and exposed to, reactive ion etching. Next, reactive-ion etching is effected by use of a radio-frequency-induced SF6 plasma.
In an experiment, the rate of reactive-ion etching was found to be about 3.6 µm per minute; in contrast, the rate of etching in hot KOH is about 1 µm per minute. It was also found that reactive-ion etching undercut the masked portion of the substrate at a rate of about 3.5 µm per minute. The diamond membrane exposed by etching of the substrate was found to be in a state of compressive stress.
This work was done by Rajeshuni Ramesham of Caltech for NASA's Jet Propulsion Laboratory. NPO-20477