Advanced Diamond Technologies (ADT) is to develop CMOS-compatible diamond-based MEMS for broadband communication applications with funds from US military organisation DARPA.

The material is not monocrystaline and has been dubbed ultra-nanocrystalline diamond (UNCD).

“DARPA is interested in increasing the reliability of MEMS devices by combining them with CMOS on the same chip,” president Neil Kane told Electronics Weekly. “You cannot do this on top of CMOS with traditional polysilicon, SiGe or SiC because the process temperature is much too high.”

The Illinois-based firm’s deposition technique, developed at the US Argonne National Laboratory, grows diamond in a 99 per cent argon, one per cent methane atmosphere which allows deposition at under 400°C. Traditional CVD diamond from one per cent methane in 99 per cent hydrogen needs over 800°C.

“It is a different chemical pathway,” said Kane. “Instead of cracking CH3, in our process C2 dimer molecules precipitate on the surface. It requires much lower energy and therefore less temperature.”

The diamond grains in each case are different: several microns across with the traditional CVD, whereas the argon atmosphere produces grains around 5nm across.

“The smaller grains mean a much smoother surface and as most atoms are close to a grain surface we can control electrical properties by doping the boundaries, and it is a little bit tougher,” said Kane.

For resonant structures, light weight and high stiffness have advantages, encapsulated by a high ‘acoustic velocity’ figure. “We recorded the highest acoustic velocity ever measured,” said Kane. “That is the highest resonant frequency for a given size of structure.”

CTO Dr John Carlisle told EW the figure was 16,200m/s, twice that of polysilicon, close to the 16,700-18,000 possible in natural diamond and more than the 11,000m/s possible with SiC processed at 1,000°C.

“If you deposited it on CMOS, you should be able to make a 2-3GHz resonator,” he said.

The aim of the DARPA funding is to put UNCD MEMS on CMOS. Previous research, that produced the 16,000m/s material, was mid-temperature diamond made under 800°C. Although this is not compatible with CMOS, “it is useful right now”, said Kane.

Argonne National Laboratory
Innovative Micro Technology
Advanced Diamond Technologies