The Ultimate Deposition Source?

Important film properties including hardness, density and stoichiometry depend on the energy of the depositing species. Recent PVD advancements have involved increasing this energy through augmented process ionization. For example, unbalanced magnetron PVD increases the ionization of the sputtering gas through a special arrangement of the magnets - the source material isn't ionized, but the reactivity of the process is enhanced. If the the source material is fully ionized, the energy of virtually every particle impinging the growing film can be precisely controlled through substrate biasing to optimize film growth energetic conditions. This is especially important when depositing diamond like carbon films where high sp3 diamond is synthesized only in a narrow window of carbon ion energy around 100 eV. Cathodic arc PVD, offering high source material ionization, has long been where other PVD technologies strive to go, but suffers from macroparticles (MPs). Filtered cathodic arc (FCA) eliminates MPs and generates a fully ionized metal or carbon plasma (ionizing some of the process gas as well) but typically suffers from low rates, complexity, large size and high cost. FCA holds great promise as a versatile PVD source for producing the highest performing thin films, but these limitations must first be overcome.

A new kind of FCA has been developed by Fluxion (fluxion-inc.com). The Radial Arc source (shown above with a carbon plasma) may allow FCA technology to achieve on the coating center floor the success that it has so far achieved mainly in the laboratory. It accomplishes this by using a novel filter geometry that lends itself to uniform high rate deposition, compact size, simplicity and low cost (less than a magnetron, including power supplies). According to Fluxion's website, the geometry of the Radial Arc FCA can be visualized by imagining a ninety degree bent-tube filter rotated around one of its two ends where the axis of rotation is the large radius (the radius of the ninety degree bend), intersecting that end of the tube (see animation at http://fluxion-inc.com/index_OurTechnology.htm). The ions travel in a radial direction out from the cathode and are carried around curved trajectories through the large open area of the filter by strong magnetic and electric fields, then directed to the substrate in a uniform distribution. The much increased open area through the filter compared to a curvilinear filter, allows for greater ion throughput (and greater resulting deposition rate). The unique geometry of the Radial Arc also provides for strong magnetic fields in a compact design, also adding to ion transport efficiency. This visualization also indicates how MPs are filtered out by eliminating line-of-sight between the cathode and the substrate. In fact, the large and abrupt angle that MPs have to navigate to escape the filter further decreases the likelihood that MPs will reach the substrate.

Filtered Cathodic Arc PVD

Cathodic-arc evaporation is a relatively simple PVD technology that produces a large flux of highly ionized vapor, valuable for depositing hard, dense, and well adhered industrial coatings. Cathodic-arc evaporation also produces macroparticles (MPs) that create defects in the films, relegating this technology to applications that are mostly insensitive to these defects, such as cutting tool coatings. Many methods have been tried over the years to filter out MPs. Although more or less successful at reducing MPs, all of these filtered cathodic-arc (FCA) sources also reduce the coating rate and area to such an extent that they are mostly relegated to the laboratory or to applications needing only extremely thin films over small areas. FCA technology is also typically quite complicated, bulky and expensive.
The present convention in FCA, the curvilinear FCA, borrows from fusion research to bend the ions through a bent tube using a magnetic field. MPs, unlike the ions, are unaffected by the magnetic field and travel in straight lines, getting caught on the walls of the tube and are thereby prevented from reaching the part to be coated. The main problem, also present in fusion technology, is that the ions are imperfectly confined and mostly don’t make it out the filter, which explains the low deposition rates inherent to conventional FCA. Curvilinear filters are also complicated to operate and typically quite large, sticking some distance out of the side of the vacuum chamber. Low deposition rates over small areas (magnetic restoring can increase coating area, but adds even more complication), difficult operation, bulky size, and high cost have prevented wide-spread adoption of filtered-cathodic-arc (FCA) technology, despite the many advantages of ion deposition.

PVD Diamond Like Carbon Thin Films

Continuing our discussion of diamond thin films, we move on to PVD diamond-like-carbon (DLC) films. The main advantages of PVD DLC over CVD diamond are lower temperature deposition (room temperature versus 400C at best for CVD), lower cost typically and a benign environmental footprint. The best PVD DLC is deposited using cathodic arc and can be 3 times harder than sputtered DLC. Cathodic arc produces carbon ions which, with careful substrate bias control, allow the ideal energetic conditions for optimizing diamond, sp3 bonding in the growing film. High sp3 bond ratio correlates with high hardness, up to about 90 GPa, or about the same hardness as CVD diamond (http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=2574&DID=118953&action=detail). Some DLC films have been reported to be even harder than natural diamond; natural diamond nano-indentors can break during hardness measurements.
The main drawback of PVD versus CVD diamond is the difficulty in growing thick films due to compressive stress. CVD "diamond-sheet" films can be 50 microns thick, compared to about 2 microns maximum for the best PVD DLC films. Numerous process modifications have been developed for the relief of stress, including post deposition annealing and substrate high voltage pulsing, but no one has yet brought a high sp3 DLC film to market that is as easy to widely implement as other more standard PVD films, such as titanium-nitride.

Ultrananocrystalline Diamond Thin Films Applications

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.

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, 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.

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, entered the market in late 2007.

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. See previous post for more on UNCD.

Ultrananocrystalline Diamond Thin Films Advance

Left: UNCD deposited onto a 200 mm silicon wafer shown on top of an uncoated wafer. Right: High-resolution transmission electron microscopy image of UNCD thin film showing nano-sized grains.

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. Advanced Diamond Technologies (ADT) was formed in 2003 to commercialize the ultrananocrystalline diamond technology developed at Argonne National Laboratory.

The UNCD innovation is rooted in the chemistry used to synthesize the material. 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. Efforts to develop smoother films led to work on nanocrystalline diamond (NCD). 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).

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). More on UNCD applications coming soon. http://www.ceramicindustry.com/CDA/Articles/Cover_Story/BNP_GUID_9-5-2006_A_10000000000000177645