What is the Ultimate Dielectric Material? Diamond Materials, Part 4Gary Kardys | August 30, 2018
Nanotechnology, microelectronics design and semiconductor fabrication have continued to shrink microelectronics and enhance the performance of microprocessors, power electronics, RF chips and other active devices. Currently available technology for passive components like capacitors and resistors is limiting further optimization. Enhanced passive devices could improve the operating characteristics of wireless and power conversion products used in military, communications, oil and gas, industrial and consumer applications. The industry requires passive devices that can handle high power densities, high frequencies and elevated operating temperatures.
5G wireless infrastructure and advanced phased array radar systems require RF resistors capable of operating above 8 GHz with power levels greater than 100 W. The U.S. Air Force and military have been interested in improved dielectrics to reduce the size and weight of passive components and increase performance because there are thousands of capacitors on a single aircraft. Passive devices are a critical element in power management and control in automotive engines, directed energy weapons, energy converters, power conditioners, electric vehicle (EV) systems, energy storage systems and industrial equipment.
The U.S. Department of Energy (DOE) has funded the development of high-temperature capacitors because downhole sensors and electronics in oil and gas operations are exposed to high temperatures. High-temperature capacitors could enable deeper exploration in higher temperature environments, reduce electronic cooling requirements and reduce device losses from overheating. Polycarbonate (PC) and polyetherimide (PEI) dielectric films have working temperature limits of 150° C, and even the fluorinated ethylene propylene (FPE) fluoropolymers and titanate ceramic dielectrics have working temperatures limited to 200° C according to data in the 2018 IET Nanodielectrics article, “Dielectric materials for high-temperature capacitors”. High dielectric strength and unexcelled thermal conductivity make diamond the ideal electrical insulation material in passive devices. Diamond dielectrics could allow passives to continue operating at higher temperatures, potentially above 400° C, with up to ten-fold reduction in weight and size.
High-Performance CVD Diamond Resistors
Smiths Interconnect is making Diamond RF Resistives, including diamond resistors, diamond attenuators and diamond terminations. Res-net Microwave has also introduced chemical vapor deposition (CVD) diamond resistors and attenuators. The resistive thin-film elements are formed on CVD diamond substrates. Diamond resistive devices have high power capability and broad frequency response in compact and lightweight packages. Diamond RF resistive devices can provide improved VSWR between stages. Diamond resistive devices can handle five times the power and ten times higher frequencies in a package one-fifth the size of state of the art alternatives using beryllium oxide (BeO) and aluminum nitride (AlN). Fabrication of high-power resistors able to operate above the S band requires reducing its parasitic electrical characteristics to a minimum.
High-Performance DLC and CVD Diamond Capacitors
Shrinking the size and enhancing the power storage capability of capacitors will also help shrink electronics and improve the performance of power conversion systems leveraging advanced gallium nitride (GaN) and silicon carbide (SiC)-based power electronics. The high energy density (above two joules per cubic centimeter) and high-temperature capability of CVD diamond capacitors provide a means for producing small, light and durable electronics products.
Twenty years ago, K Systems Corp. was awarded phase I and II rounds of Small Business Innovation Research funding to develop “A Manufacturing Technology for High Energy Density Diamond-Like Carbon Capacitors”. K Systems, Dearborn, NEC (Tokin/Kemet) and SigmaTech were involved in developing and manufacturing the diamond-like carbon (DLC) film capacitors.
In a 2011 final Department of Energy technical report evaluating various capacitor dielectric materials, Development of High Temperature Capacitor Technology and Manufacturing Capability, researchers from Hamilton Sundstrand, Brady Corporation, Steinerfilm and Dearborn Electronics, reported: “Although DLC (Diamond Like Carbon) & PCD (Polycrystalline Diamond) Capacitors have the potential of achieving size and weight reductions compared to present PC [polycarbonate] capacitors (assuming very thin supporting base metals), the required lengths of film are presently not available for large value (30 to 45 μF), 400 V capacitors. Additionally, since DLC and PCD are deposited dielectric material, on a supporting base metal, the size, weight, and resulting performance of the capacitor is dependent upon the base metal selected.”
K Systems DLC capacitor technology outperformed many older methods. The DLC coating dielectric strength of 6,500 kV/cm was better than many high-performance dielectric polymers like polytetrafluoroethylene (PTFE), polyetherimide, polyimide (Kapton) or FPE, all of which are widely used in capacitors according to several GE Global researchers in the IEEJ Transactions on Fundamentals and Materials article, Advanced Dielectrics for Capacitors. However, the DLC film dielectric strength falls far short of the CVD diamond materials. CVD diamond dielectric strengths are typically listed around 10,000 kV/cm, but some researchers and manufacturers report they have achieved dielectric strengths 30,000 kV/cm or more. The higher dielectric constant of diamond (5.6) compared to PC (3) or PFE (3.4) would translate to increased device capacitance for the same film thickness. In addition, since CVD diamond has a higher breakdown voltage of 10,000 kV/cm as opposed to PFE’s 4,700 kV/cm and PC’s 3,150 kV/cm, an even greater increase in capacitance (or decrease in size) can be achieved by going to a thinner gauge of dielectric layer or film.
Today, true diamond materials are made through CVD methods. In 2017, Wolfspeed and FemtoScience received a $500,000 ARPA-E award, “Compact, High Voltage, High Energy Density Diamond Capacitors for Power Electronics Applications”, to develop CVD diamond capacitors for power electronics and energy storage. Compact, low-cost CVD diamond capacitors could be free of life-limiting overheating under both low-frequency (high energy) and high-frequency (low equivalent series resistance) conditions. They proposed developing capacitors with incredible energy densities of 30 J/cm3. Figure 3 provides an example of FemtoSci-proposed technology to create advanced high-voltage and high-energy density ultracapacitors utilizing CVD diamond. Key steps for implementation are the development of robust electrode metallurgy and device packaging methods. These capacitors will achieve a temperature coefficient of capacitance of 10 ppm/° C, and are capable of rapid charging and discharging (less than 400 ns) over thousands of cycles.
Dielectric Diamond Coatings
CVD diamond without doping has excellent electrical insulating or dielectric properties, such as a low dielectric constant of 5.7, a loss tangent below 0.00005 at 145 GHz, room temperature resistivity of 10E+16 ohm-cm and a high dielectric strength of 1,000,000 V/cm or 10,00 kV/cm. Femto Science claims diamond can have the highest dielectric strength of any material: 30 MV/cm or 30,000 kV/cm. Of course, actual electrical resistivity and dielectric strength are greatly impacted by impurities, dopoants, structure, interconnected porosity, flaws and microcracks from thermal expansion mismatches.
The emerging nanocrystalline diamond coatings may have the toughness to overcome the microcracking problem. Dielectric CVD diamond coatings or bulk materials have great thermal conductivity and chemical inertness, corrosion resistance and extreme scratch resistance, which could provide heat dissipation and environmental protection in addition to electrical isolation. In the 2008 IEEE 2nd Electronics System Integration Technology conference paper, “Current Leakage Failure of Conformally Coated Electronic Assemblies”, the authors noted that small ceramic capacitors have occasionally been found to fail in some applications, causing significant financial losses, particularly when covered with a thick silicone conformal coating. The researchers believe that the dielectric failure occurred when residual moisture combined with flux residue and other organic or ionic contamination diffused through the conformal coating. Perhaps a nonconductive diamond film deposited on the top of ceramic packaged electronic devices in place of a conformal coating could provide corrosion and environmental protection while allowing additional heat dissipation through the protective dielectric diamond coating.
CVD diamond deposition typically occurs in the 700° to 900° C range. Argonne National Laboratory’s patented technology can deposit nanocrystalline diamond films while keeping substrates below 400° C. While a 400° C substrate temperature is an enabler for diamond-on-silicon applications, this might be too high for conventionally soldered circuit boards. Through-hole components are typically soldered at 370° C and surface mount components at 315° C.
If a diamond conformal coating process temperature were below the solder temperature, then diamond dielectric coatings might someday be viable on printed circuit boards (PCBs). Ultra-high temperature solder alloys or conductive paste circuit connections might enable the use of dielectric CVD diamond films for conformal coatings on PCBs or ceramic-packaged devices. While diamond-like carbon films do not have the dielectric properties of CVD diamond, substrates can remain at room temperature and DLC coating can still have dielectric strengths, wear resistance and environmental resistance superior to polymer films.
While substrate deposition temperatures are too high for PCB applications, diamond dielectric films might be useful as an alternative to glass or vitreous paste hermetic sealing of LTCC and HTCC-packaged microelectronic devices. A protective diamond coating might be useful in military and aerospace applications where high radiation levels would destroy conventional hermetic sealing or dielectric coatings. In any case, dielectric diamond coatings are ideal for dielectric optical mirror, lens and window applications. Currently, there is a great deal of interest in edge-cooled diamond windows for high-power microwave tubes (gyrotron) with power levels exceeding 1 MW.
Nanodiamond Filled Electrical Insulation
In 2015, ABB received a patent, Conductor arrangement with insulation for an electrical machine, for the use of nanoscale diamond-laden insulation to isolate electrical conductors in motors, generators and other electrical power equipment. The nanodiamond particle-filled layer would replace mica-resin composite. Vacuum pressure impregnation (VPI) technology is currently used by many machine manufacturers. Layers of mica tape are wound on conductors and then the mica tape layers are impregnated with thermosetting resins. Finally, the assembly is thermally cured to form the main wall insulation, a dielectric mica-resin composite. Variations of the mica-resin insulation have been in use for over 100 years. The dielectric strength of diamond is 16.9 times higher than mica, so a much thinner layer of diamond is needed for the same insulation performance. In addition, the thermal conductivity of diamond is more than 1,000 times higher than mica, which should reduce overheating in motors, generators, transformers and electrical machines. Nanodiamond filler additions can also increase the mechanical strength of a resin composite. The thermal dissipation enhancements from nanodiamond additives in dielectric transformers were discussed in Diamond Materials, Part 3.
While diamond has outstanding dielectric properties, the unique optical properties of diamond (low thermal expansion, high optical transparency and transmissivity over a wide wavelength range) hold promise for high-performance display screens, optical windows, radiation detectors and photonics devices.
Stay tuned for Diamond Materials Part 5.
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