Diamonds on film
Wafers with polycrystalline films of diamond are on the market and are being investigated for use with high-performance materials like gallium nitride; today such materials often cannot perform at their peak, because even silicon carbide substrates cannot dissipate the heat fast enough. Wafers with polycrystalline films of diamond are also being proposed as higher-performance replacements for silicon-on-insulator wafers.
Pure diamond transistors are still experimental, but NTT and others have demonstrated high-power, high-frequency versions for communications. Diamond transistors have also being proposed for next-generation collision-avoidance automotive radar systems that would operate well in adverse weather/temperature conditions, as well as for applications that are projected further into the future, such as qubit storage in quantum computers.
|Diamond RF MEMS switch (left) operates at higher frequencies than silicon but can be fabricated alongside the CMOS circuitry (right) driving it, enabling a single chip to handle both functions.|
"Diamond today is used for high-temperature, high-frequency, high-cost, niche applications where someone, such as the military, can afford the extra cost of diamond substrates," said Freeman.
The two hurdles to the commercialization of single-crystal diamond semiconductors are doping and scaling. Very few dopants have been found than can introduce the lattice defects needed to change the material from an insulator to a semiconductor.
"Silicon has whole families of dopants, like boron and phosphorus, that can be implanted into silicon to achieve certain semiconducting properties, then annealed to heal the damage to the lattice. Silicon naturally recrystallizes around the dopant atoms, whereas diamond doesn't. If you try to implant dopants and anneal diamond, the doped regions will just turn into graphite [amorphous carbon]," said John Carlisle, chief technology officer at Advanced Diamond Technologies Inc. (ADT; Romeoville, Ill.).
The scaling problem refers to the inability to grow single-crystal diamond across wafers much bigger than an inch and a half. Diamond is no match for silicon, which can be grown at the wafer scale in atomically perfect single-crystalline monolayers as wide as 8 inches or more.
"Tremendous engineering challenges remain to making single-crystal diamond films that cover an entire 200-millimeter [8-inch] wafer," said Carlisle.
The diamond wafers being sold today by ADT and sp3 Inc. (Santa Clara, Calif.) are mostly used in microelectromechanical system (MEMS) applications, for which their ultrahardness is valued, and in apps that require the thermal dissipation performance of silicon carbide wafers, but at a much lower price point. "We are pricing our diamond-coated wafers at about 25 percent of the cost of silicon carbide wafers," said Dwain Aidala, president of sp3.
ADT and sp3 have sidestepped the doping and scaling problems facing single-crystal carbon films by instead growing wafer-scale polycrystalline diamond films. Called microcrystalline diamond by sp3 and ultrananocrystalline diamond by ADT, these films use grains of carbon as small as 5 nm in diameter (about 10 carbon atoms wide) and consisting of just 20 to 30 atoms each.
"Nanocrystalline diamond allows us to solve both the doping and scaling problems [seen] with single-crystalline diamond," said Carlisle. "It's not perfect, but by and large we have captured the best properties of single-crystal diamond but without its drawbacks. For instance, we have successfully deposited our nanocrystalline diamond onto 300-mm (12-inch) wafers in our lab," said Carlisle. "As a result, now we can make layered structures that interleave our diamond films anywhere in a CMOS semiconductor stack."
ADT's ultrananocrystalline diamond (UNCD) is naturally insulating but can be made highly conductive by doping it with nitrogen. By situating themselves between the nanoparticle grains rather than intruding on the carbon lattice itself, the foreign atoms do not tempt the crystalline carbon into reforming into graphite. By adding dopants and altering the deposition process itself, the electrical conductivity of UNCD films can be changed by more than eight orders of magnitude (100 million to 1).
The company is also developing diamond under a Defense Advanced Research Projects Agency (Darpa) contract for MEMS applications, where the material can extend frequency performance into the gigahertz range (silicon MEMS devices are limited to megahertz) and offer long-term durability.
"Diamond has all the properties that you want for MEMS; its very high stiffness enables it to resonate at very high frequencies, and it has a very stable surface that is immune to oxidation," said ADT's Carlisle.
Next month, Darpa will evaluate its Harsh Environment Robust Micromechanical Technology (Hermit) program to fabricate diamond films using ADT's ultrananocrystalline process, which was perfected at Argonne National Lab. For Hermit, Darpa program manager Amit Lal enlisted the help of Innovative Micro Technology (IMT), which formed ADT's diamond into a MEMS device; MEMtronics, which designed the RF switch; and Peregrine Semiconductor, which fabricated the CMOS drivers atop a silicon-on-sapphire wafer.
Emboldened by the successful implementation of an RF phase shifter for Darpa, contractors are now embarking on their own development efforts to repackage diamond MEMS devices on CMOS into RF modules for consumer devices.
"Our vision is to combine several different RF oscillators, filters and switches into a single-chip solution for portable wireless devices like smart phones and smartbooks," said Carlisle, contrasting the approach with "what's done today, which is to use 30 different suppliers."