In the past decade, major advancements in the area of diamond thin film technology have been quietly taking shape and building a promising case for the application of diamond thin films in hi-tech products. This is a major shift for the field, which peaked in 1990s and then slowed down for a long time, due to a number of technological hurdles. The hurdles include large area synthesis, high substrate temperatures, high surface roughness, and difficulties achieving efficient n-type doping, which ultimately affected commercial interest in this material. However, continued academic interest and research in this area kept the field alive.
In 1998, researchers at Argonne National Laboratory invented a special form of diamond with very small grain size (2 – 5 nm), low as-grown roughness (4 – 7 nm), and high sp3 content (98 %) with mechanical and tribological properties comparable to that of single crystal diamond. Argonne researchers coined the term “ultrananocrystalline diamond” (UNCD) to name this material because of the small grain size. Over the years, a number of interesting properties of UNCD were discovered, such as semimetallic electrical conductivity (when doped with boron or incorporated with nitrogen), the lowest adhesion energy (10 mJ/m2) and friction coefficient (0.007) in self-matted configuration, along with a high hardness (98 GPa) and high Young's modulus (980 GPa), and excellent biocompatibility with resistance to biofouling [1] and [2]. These properties enabled the development of a number of applications ranging from the use of UNCD as a robust electrode for water purification at the macroscale to reusable templates for the production of nanowires at micro/nanoscales [3]. Also, it could be used as a wear resistant coating for mechanical face seals at the macroscale [4] and as a wear resistant tip for atomic force microcopy, for better imaging at the nanoscale [5]. The excellent biocompatible properties of UNCD have now opened up an entire new field, exploring UNCD coatings not only for biomedical applications such as bio-implants, and bio-inert electrodes, but also for developing bio-sensors to detect harmful pathogens in water. Some of these applications have already been commercialized by Advanced Diamond Technologies Inc., which was founded based on the research carried out at Argonne on UNCD.
Recent research carried out at the Center for Nanoscale Materials, Argonne National Laboratory demonstrated a wafer scale process to deposit UNCD thin films at 400 °C and its use in developing complementary metal oxide semiconductor (CMOS) compatible RF-MEMS switch technology [1] and [6] In this case, it was observed that UNCD works as an excellent leaky dielectric material, drastically reducing the dielectric failure that generally occurs with silicon nitride (used conventionally) due to the build-up of charge over time. Additionally, a chemically inert UNCD surface provides low adhesion, reducing stiction related problems and thus increasing the performance lifetime of these switches by orders of magnitude. The low temperature UNCD growth process developed at CNM was further modified to increase the grain size from 2 – 5 nm to 100 – 200 nm by changing the growth chemistry, enhancing the thermal conductivity of UNCD to a reasonable level [7]. The combination of moderate grain size with lower deposition temperature allowed direct integration of UNCD with important semiconductor materials, such as GaN used in high power electronics for efficient thermal management [7]. With the advances in chemical mechanical planarization technology, it is now possible to achieve surface roughness of UNCD films down to less than a nanometer level eliminating all problems associated with the high roughness of diamond films. The low roughness of UNCD films along with the high thermal conductivity, and low trap density for charges makes it a unique substrate material for developing high performance electronic devices [8]. Recent research carried out by the company AKhan Technologies Inc towards achieving n-type doping in nanocrystalline diamond (NCD) looks promising.
Looking back on the progress that has been made in CVD-diamond technology in the last 20 years, it is clear that the technology is now mature enough and already making its way towards the market through some specialized products. One can argue that in terms of commercial success, diamond thin films have even surpassed other promising carbon materials such as carbon nanotubes and fullerene. It may not be an exaggeration to say that the diamond age is not a distant dream any more but a close reality, that is just around the corner.
Further Reading
[1] Sumant et al. MRS Bulletin, 35 (2010), p. 281
[2] Sumant et al. Phys Rev B, 76 (2007), p. 235429
[3] Seley et al. ACS Appl Mater Interfac, 3 (2011), p. 925
[4] Sumant et al. Tribology Transact, 48 (2005), p. 24
[5] Liu et al. Small, 6 (10) (2010), p. 1140
[6] Goldsmith et al. IEEE Intl Microwave Symp Dig (2010), pp. 1246–1249
[7] Goyal et al. Advan Funct Mater, 22 (7) (2012), p. 1525