Diamonds Are a Technology's Best Friend

This scanning electron microscope image shows a released and suspended fixed-fixed 1st overtone mode radio frequency (RF) resonator made from Ultrananocrystalline Diamond (UNCD) Aqua 50. Compared with resonators created out of silicon, UNCD resonators will oscillate at two times higher frequencies compared to all other materials becauseof UNCD’s high acoustic velocity. Researchers will use this test structure and others like it to evaluate the performance of UNCD resonators in the megahertz range suitable for mobile telephone applications.

Research into thin films is producing upgrades in a range of products from communications systems to flat-screen displays.

Electronic devices across an array of fields may soon experience major improvements because of advancements in diamond film technologies. The material results in the enhanced functioning of various technological tools, and organizations from the military to the medical community could reap the benefits.

The development and application of ultrananocrystalline diamond (UNCD) thin films changes how technology manufacturers use diamonds because UNCD is less expensive than other types of diamond and can be applied in different ways. Researchers have been growing diamonds on films since the 1970s, so the films could be applied to other products to improve their functioning. To grow the substance, researchers combine methane and hydrogen, creating plasma. In microwave plasma systems, scientists break the molecules and create CH3 (one carbon atom and three hydrogen atoms), which bonds with carbon in what is known as sp3 hybridization. “That is the bonding for diamond, which is the hardest substance known on Earth,” explains Dr. Orlando Auciello, senior scientist at Argonne National Laboratory. He is the principal investigator for work on UNCD. The size of that early diamond grain produced four decades ago was 1 micrometer, and it had a rough surface.

Researchers began increasing the amount of methane used in the diamond-growing process and developed a substance known as nanocrystalline diamond, with a grain size reduced to between 30 and 100 nanometers. 

In the early 1990s, Dr. Dieter Gruen, a scientist at Argonne, began experiments to grow diamond without using hydrogen. Instead, he used the C-60 molecule combined with argon, a common, inexpensive, inert gas. The process yielded UNCD with grains from 2 to 5 nanometers in size, a billionfold smaller than grains in traditional diamond film.

Auciello then started to work with argon and methane instead of the expensive and complicated C-60. With the argon/methane combination, Auciello and his colleagues produced UNCD thin films. Using argon and methane is less expensive than using C-60 and methane and safer than using hydrogen and methane. In addition, UNCD is much smoother than its earlier counterparts.

Creating diamond is a difficult process; researchers have to mix gases and heat them in a reactor. Scientists obtain the chemicals to grow UNCD out of a plasma made by combining the methane and argon inside a reactor. Diamond is not the most stable form of carbon; that form is graphite. In nature, diamonds are created in the Earth under high pressure and turn to graphite unless earthquakes or volcanic eruptions expose them.

UNCD has many of the important characteristics of diamond as well as unique characteristics not found in carbon-based substances. As with traditional diamonds, UNCD is hard and inert, and it conducts electricity. Its special properties include its mirror smoothness and capability for low-temperature synthesis, which allows it to be attached to materials that can be damaged by heat. The material is hard and strong, adheres to many surfaces, is hydrophobic and has low film stress. UNCD thin films are bio-inert, so the material is suitable for many medical needs.

The films also have applications for the U.S. military. The Defense Advanced Research Projects Agency (DARPA), Arlington, Virginia, has awarded Argonne and its partners a contract to advance next-generation broadband communication devices with UNCD thin films. The substance’s properties enable microelectromechanical systems (MEMS) to integrate directly with silicon microchips. The result is faster, more reliable wireless communications.

The contract is now in Phase II, during which scientists have to demonstrate a resonator with certain qualities. When that is achieved, the goal of Phase III will be to create a prototype. During the first phase, researchers determined that UNCD exhibits the highest known acoustic velocity of any material. The higher the acoustic velocity of a substance, the higher the resonator frequencies it permits. In addition to these frequencies, UNCD can stand up to the harsh environments in which military troops often operate, so products made with the material provide more robust and reliable broadband communications. “The military, like everyone else, wants more bandwidth,” says Dr. John Carlisle, chief technical officer, Advanced Diamond Technologies Incorporated.

Four organizations have teamed on the DARPA project: Argonne National Laboratory, Argonne, Illinois, which provides the fundamental and applied science on the patented UNCD film technology; Advanced Diamond Technologies, Champaign, Illinois, a spinoff company from Argonne that is developing UNCD thin films for various applications; Innovative Micro Technology Incorporated, Santa Barbara, California, which has a large and well-equipped independent MEMS fabrication facility and provides MEMS services from design through production; and the University of Wisconsin–Madison, which has advanced microfabrication facilities at the Wisconsin Center for Applied Microelectronics and novel atomic force microscopy tools to characterize UNCD-based MEMS device performance. Auciello and Carlisle founded Advanced Diamond Technologies. Carlisle formerly was the chief technology officer at Argonne. Auciello serves as a technical adviser to Advanced Diamond Technologies but is not an employee.

Carlisle states that what the military is considering is essentially a mobile phone, but one with a much higher bandwidth and the ability to work in harsh environments. The way in which UNCD improves MEMS and other applications is similar to how a tuning fork operates. Diamond vibrates at a naturally higher frequency than any other material of the same dimensions: 3 gigahertz, or 3 billion cycles per second.

Cell phones and certain other electronic tools already operate at those high frequencies, but the team working on the UNCD films is trying to create a technology directly on a complementary metal-oxide semiconductor (CMOS) chip. With the technology located on the CMOS chip instead of at another place in the device, the wires that connect the chip and tuning fork would be only micrometers long.

“That’s important because the signals the military wants to send would be garbled on a long wire,” Carlisle explains. MEMS-on-CMOS direct integration is a goal of the DARPA project.

Another important facet of UNCD is its ability to grow uniformly over a large area such as a 6- to 8-inch silicon wafer. The wafers are single crystal pieces of silicon sliced from a much larger piece of silicon. “These wafers are the basis for all the Silicon Valley stuff that you know and love,” Carlisle states.

Dr. John Carlisle (l) and Dr. Orlando Auciello are developing UNCD films for a variety of purposes. The scientists create the substance by mixing argon and methane instead of methane and hydrogen, which creates a more volatile situation.

In wet environments, silicon surfaces oxidize and collect trash and water because of silicon’s hydrophilic properties. Diamond resists water and so avoids those problems. The smooth surface of diamond also provides a better quality factor, known as the resonator or the Q; a better Q means the better a resonator vibrates at one single, sharp frequency. For instance, if a radio could not tune to one exact frequency, then listeners would hear overlapping stations. With a higher Q, the military could have signals in places where it does not have signals today.

In current MEMS technology, transistors are made of silicon. However, because of its hydrophilic qualities, the cantilever, which is the part that resonates, can become stuck on the bottom. Auciello says this poses a serious problem that the diamond films can overcome. “We have demonstrated that we can develop a whole new MEMS technology based on UNCD,” he states. For this solution, scientists coat a silicon wafer with the UNCD thin film then perform photolithography and reactive ion etching. Once they define the cantilever, they etch all the way down to the silicon. They then immerse the coated chip in hydrofluoric acid and etch the silicon underneath. The acid cannot attack UNCD. Finally, the scientists release the beam structure, which can vibrate through other mechanisms.

“Diamond, in principal, is the best for everything,” Carlisle says. “It’s nature’s extreme material.”

In the past, the cost of diamond prohibited its use in devices that could have functioned better with the material. With the use of the less expensive gas argon to grow UNCD, diamond technology may soon be applied to everyday products. “What [people] need to know is that diamond isn’t a precious material anymore,” Carlisle explains. “It’s valuable, but it’s affordable.”

In fields outside the military, scientists are using UNCD to bring sight to the blind. As part of a U.S. Department of Energy-funded program involving five laboratories and three universities, personnel are developing artificial retinas for people who become blind from retina degeneration.

Images are formed in the brain after the eye captures light. Many people become blind when the photoreceptors in the retina die from genetic causes. The idea behind the Department of Energy project is to place a microchip on the retina that would receive radio frequency pulses from a camera mounted onto a pair of glasses. The microchip would collect the signal, and the pulses would go through the optical nerve to the brain to produce images.

Current microchips cannot be implanted in the eye because the silicon in the chip would corrode in the eye’s saline solution. Auciello explains that the chips currently are implanted on the side of the head near the eye, connected by wires to a device in the retina. “We’re exploring using UNCD as a coating to case the silicon chip to make it bio-inert and biocompatible,” he says. By coating the silicon in UNCD, the chip would resist water and saline, and the eye would not recognize that a chip has been inserted.

Artificial hips offer another medical application for UNCD. Coating each hip with UNCD would prevent it from wearing down, so people could potentially need only one replacement in a lifetime. Developers also could use the material to make biosensors that identify human blood sugar levels or that detect biopathogens for homeland security purposes.

Other uses for the diamond films include television and computer displays and mechanical pumps. Companies in the electronics field have been working on flat-panel displays in which images are formed mostly by liquid crystals.

Televisions require cathode ray tubes and filaments, and they heat up to high temperatures. They also need an electron accelerator. Companies are studying the use of electric field emissions to produce the flat-panel images. In this method, the electrons travel through space using a thinner equivalent to a cathode ray tube. This approach saves energy. It has been demonstrated but is too expensive for widespread use.

Auciello asserts that developers could use UNCD to lower voltage, producing a large amount of electrons that would create the electric field emissions image. This could work because applying volts to UNCD causes the material to release low-voltage electrons.

In addition, scientists have demonstrated that using UNCD for mechanical pump seals diminishes the friction from the rotating shaft to almost eliminate wear and to reduce the torque for the pump, saving energy. Auciello estimates energy cost savings of up to 20 percent using the diamond material.

 

Web Resources
Argonne National Laboratory: www.anl.gov
Advanced Diamond Technologies Incorporated: www.thindiamond.com
Defense Advanced Research Projects Agency: www.darpa.mil
Innovative Micro Technology Incorporated: www.imtmems.com
Wisconsin Center for Applied Microelectronics: www.engr.wisc.edu/centers/wcam