If natural diamond were inexpensive and could be integrated into products as easily as other materials, it could be used in countless applications. A solution to this decades-old problem was recently discovered-diamond films made by a new low-pressure vapor process make diamond a viable choice for engineers. In particular, thin film diamond can now be integrated with ceramics for friction/wear applications, thermal applications and microsystems.

Thin film diamond chemical vapor deposition (CVD) technology was developed about 30 years ago. Despite advances over the years, CVD diamond has failed to meet lofty industry expectations based on the perfection of natural diamond. Instead of becoming the material of choice for demanding applications, thin film diamond has been commonly referred to as the material of last resort; engineers consider CVD diamond only if every other commercial material fails to do the job.

In addition to cost, a number of technological problems, such as poor reproducibility, lack of mature deposition technology, relatively small deposition areas and poor film properties, have restricted the application of thin film diamond to cutting tools, heat sinks and other niche markets.

Over the past two decades, research at Argonne National Laboratory led to the discovery of ultrananocrystalline diamond (UNCD^®), a new technology that overcomes previous limitations related to thin film diamond.* The UNCD innovation is rooted in the chemistry used to synthesize the material.

*Advanced Diamond Technologies (ADT) was formed in 2003 to commercialize the ultrananocrystalline diamond technology developed at Argonne National Laboratory. ADT is the exclusive licensee to Argonne's portfolio of patents for synthesizing and using UNCD, and has received generous support from the National Science Foundation's Small Business Innovation Research program, the U.S. Department of Energy, the Defense Advanced Research Projects Agency, and the State of Illinois' Department of Commerce and Economic Opportunity.

Developing a Smoother Alternative

Figure 1. High-resolution transmission electron microscopy image of UNCD thin film.

In the past, the growth chemistries used to synthesize pure CVD diamond resulted in rough films characterized by very large diamond grains (microns in size) and weak, low-angle grain boundaries. These large-grained, microcrystalline diamond (MCD) films were nearly impossible to integrate with other materials, and the palette of compatible substrates was limited. Applications for such films tended to require thick, freestanding pieces of diamond that took days or weeks to synthesize and were extremely expensive. They were also rough on one side and consisted of graphitic carbon on the other, requiring extensive post-processing to be usable.

Efforts to develop smoother films led to work on nanocrystalline diamond (NCD). The straightforward modification of the chemistries used in microcrystalline material to make NCD resulted in films that were combinations of the diamond and graphitic phases of carbon, but these films exhibited inferior properties, such as greatly reduced hardness, modulus and fracture strength. Consequently, NCD developed a poor reputation in the diamond research community.

Overcoming the inferior qualities of MCD and NCD demanded a new deposition technology. Growing diamond requires techniques to generate chemical species that are unstable under normal conditions. The role of the excitation source (i.e., plasma) is to heat up the reactants to temperatures where the decomposition of methane (CH₄) and hydrogen (H₂) can form reactive species.

Argonne researchers discovered that by adding argon gas, along with methane and hydrogen, to the vapor mixture, the radicals generated during growth changed and yielded diamond films consisting of approximately 5 nanometer grains without any graphitic phases (i.e., phase-pure, diamond-bonded carbon with high-energy, atomically abrupt grain boundaries).

Ultrananocrystalline diamond was the name given to this film to distinguish it from its predecessor, NCD film. Because carbon atoms are bonded together in grains that are about 20 carbon atoms in size, UNCD is as hard and stiff as natural diamond while tougher and stronger than typical MCD films. UNCD's film stress is also significantly lower, and by adding impurities such as nitrogen, boron or hydrogen to the grain boundaries, the properties of UNCD can be adjusted depending on its application (see Figure 1).

Since 5 nm grain UNCD is phase-pure, diamond-bonded carbon, it retains the desirable characteristics of natural diamond, including hardness, modulus, refractive index, acoustic velocity and surface chemistry. UNCD's electronic, thermal and optical transport properties, however, are different from natural diamond. It also has a low thermal conductivity compared to single crystal diamond (10-20 vs. ~2200 W/m°K) and is not optically transparent.

From an electronic structure point of view, UNCD has diamond's bandgap (5.5 eV), but the bonding states within the grain boundaries create a material that is more electrically conductive with lower electron mobility. UNCD features a number of unique properties, including low as-deposited film roughness, low stress, higher fracture toughness and higher strength compared to natural diamond. Unlike previous NCD technologies, UNCD films can be grown at lower temperatures (400°C compared to 600-900°C). All of these differences result in a thin film diamond that can be easily integrated with other materials, such as other thin film technologies that are the basis for microelectronics and microelectromechanical systems (MEMS).

Products and Commercialization

Figure 2. UNCD deposited onto a 200 mm silicon wafer (left) compared with an uncoated wafer (right).

A family of materials based on UNCD  has been developed that offers an array of adjustable properties to make it suitable for a range of thin film diamond applications. For example, very smooth films can have transport properties close to natural diamond, yet retain the superior ability of UNCD to integrate with other materials. It is now possible to make electrically or thermally conductive films that are extremely smooth with low stress for use as thermal coatings.

Furthermore, advances in deposition technologies have enabled the production of these films on substrates of up to 300 mm. One commercially available UNCD series of films on silicon wafers** allows researchers and industrial developers to evaluate these materials for potential applications (see Figure 2). In addition, the first UNCD-based diamond-on-insulator wafers⁺ were developed specifically for MEMS and electronics application development. UNCD also integrates with many other thin film ceramic materials, including SiC, SiN, AlN, ZnO and PZT.

The list of diamond-related products is endless, but the market potential varies dramatically, ranging from niche applications like synchrotron X-ray detectors to the billion-dollar market of radio frequency (RF) electronics. Key components of the UNCD technology will be advanced to address the major opportunities in MEMS, biomedical applications and other industries while allowing UNCD, both as a material and a technology, to gain acceptance and to disprove the previous mindset that diamond is both expensive and unreliable.

Currently, three specific applications enabled by UNCD are under development. The first is wear-resistant, low friction coatings for mechanical components, including mechanical seals for fluid pumps. UNCD films as thin as 1 micron can change the performance of state-of-the-art silicon carbide seals and dramatically reduce the friction and wear at the seal face, increasing the lifetime of the pump for applications in chemical refineries, ethanol production, petroleum exploration and pharmaceutical processing. Since mechanical seals are found in most fluid pumps, it is estimated that reducing friction could save trillions of BTUs of energy annually. The same UNCD films can be used as a tribological coating in other industrial settings.

**UNCD Aqua series of films deposited on silicon wafers (DoSi^TM)
⁺UNCD DOI^TM

Figure 3. Scanning electron micrograph (SEM) image of a released and suspended fixed-fixed 2nd overtone mode RF resonator made from UNCD Aqua 25. Image courtesy Innovative Micro Technology (IMT), Santa Barbara, Calif.

The second important application area for UNCD is as a structural material in MEMS, including AFM probes, RF MEMS filters, oscillators and switches. These applications leverage the greatest number of diamond's superlative bulk and surface properties, since the performance and long-term stability of MEMS devices depend on the chemical stability of the exposed surface.

For RF MEMS, such as resonators (see Figure 3), UNCD acts like a tuning fork, vibrating at a set frequency that cannot vary with time, temperature or other environmental conditions. UNCD, like natural diamond, has a chemically inert, hydrophobic, low stiction surface that allows devices to function without the need for expensive die-level hermetic packaging.

Figure 4. Scanning electron microscopy image of an UNCD atomic force microscopy probe.

By leveraging the high acoustic velocity and surface stability of UNCD, devices for X- and Ka-band (2-20 GHz) wireless communication systems can be developed that allow for smaller, more energy efficient and less expensive RF front-ends for radios in mobile phones, base stations and military applications. UNCD-based atomic force microscopy (AFM) probes,^· which are simple forms of MEMS devices, will enter the market in late 2007 (see Figure 4).

^·NaDiaProbes^TM

Figure 5. Schematic of a UNCD-based MEMS biosensor.

The third application area includes bio-implants and sensors, with the goal of creating functional devices that integrate both passive and active UNCD elements combining diamond's bio-inertness and bio-compatibility with the ability to covalently immobilize biomolecules on the surface (see Figure 5). Active electrochemical-based sensors using conductive UNCD thin films can enable implantable devices that conduct real-time monitoring of blood chemistry (e.g., glucose, alcohol, cholesterol). This advancement will enable a new generation of biosensors that work in real-time in devices that are both compact and light enough to wear as jewelry. Imagine the life-changing and potential life-saving impact of wearing a "watch" that automatically monitors and administers insulin continuously via a wireless link to an implanted UNCD-based biosensor.

Such biomedical applications will take additional effort to overcome many fundamental technical challenges. However, diamond has finally come of age in a platform technology suitable for broad integration into numerous applications, and UNCD is being developed into commercially available products to turn the idea of diamond for use as an engineering material into a reality.

For additional information regarding UNCD, contact Advanced Diamond Technologies, Inc., 429 B Weber Rd. #286, Romeoville, IL 60446; (815) 293-0900; e-mail info@thindiamond.com; or visit www.thindiamond.com. Argonne's website is located at www.anl.gov.