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ADVANCED CAPACITORS

ADVANCED CAPACITORS

Capacitors continue to represent a critical component in electronic devices utilized for electrical energy storage and switching.  Today’s capacitors needed in conjunction with advanced electronics, are facing limitations either in energy density, thermo-mechanical stability, and/or volume efficiency.  The expertise and innovation in thin films’ growth and characterization, material synthesis and processing, device conception, fabrication, and lab-on-chip integration has lead to the development of practical solutions that materialized into proven technologies and protected intellectual property (IP) ready to be embraced for a path to commercialization. Two distinctly different categories of advanced-capacitor materials and technologies have been developed: High Temperature Boron Nitride Based Capacitors and Capacitors Based on Metamaterials for Giant K Dielectrics. Each technology capitalizes on its unique advantages to provide the optimal and practical solution for users and integrators.

High Temperature Boron Nitride Based Capacitors

High-dielectric constant ceramics, widely used because of their high volumetric efficiency, typically show a large decrease in capacitance at high temperature. Capacitors made with lower-dielectric constants materials, glass-ceramic, mica, and other less common dielectric materials exhibit less change and appear to be usable at temperatures > 400 oC. However, their volumetric efficiency is low, and some of them are only laboratory items. Thin film capacitors, based on relatively stable inorganic dielectrics such as SiO2 and Si3N4 (or a combination), have shown potential for high-temperature applications. Chip MOS capacitors have been used in commercial hybrids at up to 200 oC.

Despite the extraordinary properties of BN (high bulk modulus, high thermal conductivity, low

density, high electric breakdown field strength, resistance to radiation damage, and large bandgap), the research activity on this material as a dielectric for high temperature capacitor applications is low. There is, however, a renewed interest in this compound as a hard coating material, competing comparatively with diamond like thin films, as a material for UV optoelectronic devices for ranges not accessible with GaN or SiC alloys, and for super high temperature devices.

Employment of thin BN layers, that previously were proven to have superior, mechanical, electrical, and temperature resistance [1, 2], for fabrication of advanced ceramic capacitors can potentially provide the following unique features and benefits:

  • High frequency (applications in rf : cell phones and pulsed power)
  • High energy density storage (> 10 MJ/m3 );
    (Applications: ignition system, electric vehicle, powered equipment)
  • High breakdown voltage (> 3000 V) (applications: power supplies);
  • Extend the use of CC’s for high temperature applications (temperatures up to 500 oC or more);
  • Low loss (< 0.15% ) (pulsed mode applications);
  • High specific energy ( >3 kJ/Kg);
  • High reliability.

High reliability.

The objective of this project [3] was to develop boron nitride based capacitors for high temperature high-energy density applications. In the course of the project efforts were devoted to demonstrate the feasibility of fabricating multilayer ceramic capacitor chips based on insulating thin boron nitride layers and conductive aluminum or tantalum nitride as internal electrodes.

Ion assisted physical vapor deposition (PVD) technique to generate the thin film materials on titanium nitride ceramic plates. Beside the boron nitride dielectric layer and metal electrodes, the capacitor structure processing included fabrication of diffusion silicon oxide barrier layers generated by a plasma enhanced chemical vapor (PECVD) deposition method. These intermediate layers were placed to prevent diffusion of the metal electrode through the BN active dielectric layer during either high temperature growth or post growth thermal annealing.

Modeling of the capacitor structures, performed for various contact and barrier materials, indicated several expected benefits resulting from using BN as an active capacitor layer [4]. The achieved capacitance dependence of the frequency and temperature of the fabricated multilayer ceramic capacitor (MLCC) structures is shown in the picture below:

Replacement of the Al and TiN electrodes with Ta-Al-Ta electrodes improved the capacitor characteristics. Based on the results, the Ta-Al-Ta sandwich-type electrode offers the best combination of low series resistance at higher frequencies with increased thermal resilience. The use of Al instead of Au for the high conductivity layer helps reduce the stress build-up in the sandwich electrodes and multiple capacitor layers. The Ta layers are effective at containing the diffusion-prone Al metal at high temperatures while preserving the high conductivity of the Al metal. From the annealing experiments, the estimated operational lifetime (C drops to ½ initial value) was approximately 4 years at a temperature of 320 ºC for a Ta-Al-Ta based capacitor device.

Finally, capacitor structures with Ti-Al-Ti layers with SiO2 barrier layers fabricated for high-temperature SiC transistors as matching capacitors, indicated ability to work at temperatures above 400 ºC:

Two patents resulting from this technology [5,6] describe a method for storing energy in a capacitor, that includes connecting a first conductor to a first electrode. A second conductor is connected to a second electrode. The second electrode is separated from the first electrode by a dielectric layer. The dielectric layer includes a layer of boron nitride, BN. The conductivity of the dielectric layer is lower than the conductivity of the first electrode or the second electrode. A voltage of at least 5 volts is applied between the first electrode and the second electrode. The voltage is applied be means of the first and second conductors.

In order to improve capacitor characteristics, prototype capacitors with boron oxynitride (BON) as dielectric have been fabricated. Preliminary results on the application of this material indicate a very small variation (~3%) of capacitance over a range of frequencies (10 KHz – 2 MHz). The measured capacitance values from different spots at 10 KHz were about 0.2 nF. This is for a typical 200 nm thick B1.0O0.5N0.5 layer and an area of 1 mm2 for the 100 nm thick Ti electrode. The variation in capacitance over a range of temperatures 25°C-400°C was about 13% for a 300 nm thick B1.0O0.5N0.5 layer with similar footprint and metal electrodes. The dielectric constant (permittivity) of the BON thin films is found to be 5.5 – 8 as compared to 3.9 of SiO2. The associated loss is found to be smaller when compared to Boron Nitride of equivalent physical thickness and exhibit good linearity in terms of electrode area.

Boron Oxynitride has effectively eliminated the need for a diffusion barrier and has proved to

be self-sustaining at high temperatures. The advantages of this technology are described in a US Patent Application [7], presenting a method for storing energy in a capacitor, that includes connecting a first conductor to a first electrode and a second conductor to a second electrode. The second electrode is separated from the first electrode by a dielectric layer. The dielectric layer includes a layer of boron oxynitride, BON. The conductivity of the dielectric layer is lower than the conductivity of the first electrode or the second electrode. A voltage of at least 5 volts is applied between the first electrode and the second electrode by means of the first and second conductors.

Capacitors Based on Metamaterials for Giant K Dielectrics

Capacitors based on polymers-ceramic composites because combine the processability of the polymer with the high dielectric constant of the ceramics. However, if the dielectric constant of the polymer-ceramic composite has to be maintained high then it has to be heavily doped with ceramic fillers, but if the polymer is doped at a higher level there is a problem of poor adhesion [. Alternatively, embedding metal nanoparticles (NP) instead of ceramic dielectric in polymer matrix seems to be a very effective way to enhance the dielectric performance of the nanocomposite near the percolation threshold along with maintaining a good adhesion to the substrate.

Initially, a novel laser  processing technique was explored to produce nanodielectric films, which are based on polymer coated metal  nanoparticles [8]. Poolyacryl-coated gold nanoparticles (Au NP) were sucsess fully processed as the nanodielectric material. First, Au NP are synthesized  by laser ablation of bulk Au in liquid, e.g., ethanol.  This colloidal solution wass added to a liquid monomer  for further polymerization. It wass assumed that both the monomer and resulting polymer are transparent at the laser wavelength. Then the mixture wass exposed to laser radiation, which was absorbed by Au NP. In a  sense high temperature is equivalent to UV photons needed for polymerization. Then the laser exposed mixture was centrifuged and washed in ethanol to remove the residuals of the non-reactant monomer. The polymer-coated Au NP composite was spin-coated onto a nickel foil serving as the bottom electrode and aluminum was

used as the capacitor top electrode.

The objective of the next project [9] was to increase capacitance of the nanocomposite and, therefore, the storage capability without compromising the inter-particle spacing, which is the critical dimension for the onset of charge transport by having as many as possible core-shell capacitors uniformly distributed in the host polymer. The best way to achieving this is to have each metal nanoparticle coated with the polymer and process a large enough quantity of such structures to make a prototype capacitor slab.

In the course of the project experimental and commercially available nanoparticles  were entrained in a broad spectrum of host polymers, in a very innovative way that outperforms currently sought nanodielectrics. Scaling up this technology without losing its unique and valuable properties was accomplished through a wet chemistry route using laser for selective UV polymerization, where each metal nanoparticle is coated with a polymeric shell, thus converting the process to a continuous flow to form super-capacitor slabs.

The  achievements of this project include: fabrication of capacitors using silica-coated Silver (Ag) nanoparticles, fabrication of capacitors using PVP-coated Ag nanoparticles, characterization of capacitor samples fabricated from Ag-SiO2 core-shell nanoparticles, optimization of minimum required polymer quantity and fraction of loading (FOL).  The enhancement of the effective dielectric constant of the nanodielectrics by a factor of about 6 (k > 40) was achieved by using Ag nanoparticles dispersed in PVP, representing a good match with the k value of 58, as predicted by percolation theory.  An optimized method was designed to increase the k value and also to strengthen the cross link between the nanocomposite solution and the Si wafer.

The LCR characteristics were measured for a range of frequencies from 20 Hz to 10 MHz and exhibited less than 20% degradation over that range.  The measured breakdown voltage was >156 V/µm.  Furthermore, a new procedure was implemented to increase the FOL of nanoparticles in the polymer, by using very lower concentration of nanoparticles.  As result, the FOL in a polymer matrix was increased up to about 0.08 %, without agglomeration of nanoparticles.  Devices fabricated with the new FOL were tested and exhibited k value close to 70. Capacitor prototype devices have been modeled [10], fabricated, characterized, and packaged:

REFERENCES

[1] Boron Nitride (BN) – Properties and Information on Boron Nitride. http://www.azom.com/article.aspx?ArticleID=78

[2] N. Badi , A. Tempez , D. Starkov , N. Medelci , A. Bensaoula, J. Kulik , S. M. Klimentov , S. V. Garnov , V. P. Ageev , M. V. Ugarov , S. Lee, S. S. Perry, K. Waters and A. Shultz (1997) Boron Nitride Materials for Tribological and High Temperature High Power Devices. MRS Proceedings, Volume 495, 359.

https://www.cambridge.org/core/journals/mrs-online-proceedings-library-archive/article/boron-nitride-materials-for-tribological-and-high-temperature-high-power-devices/B896C6EAA872A47B100F3EBA97F7C294

[3] Boron Nitride Capacitors for Advanced Power Electronic Devices, N.Badi (Principal Investigator) (2006) DOE SBIR Contract: DE-FG02-05ER84325.  https://www.sbir.gov/sbirsearch/detail/197382

[4] N. Badi, C. Boney, and A. Bensaoula (2004) Self-packaged Boron Nitride Capacitor for High Temperature Applications. Journal of Microelectronics and Electronic Packaging: October 2004, Vol. 1, No. 4, pp. 217-224. http://www.imapsource.org/doi/pdf/10.4071/1551-4897-1.4.217

[5] A. Bensaoula and N.Badi (2003) Capacitor and method of storing energy. US Patent number: 6570753. http://patents.justia.com/inventor/abdelhak-bensaoula?page=2

[6] A. Bensaoula and N.Badi (2005) Capacitor and method of storing energy. US Patent number: 6939775. http://patents.justia.com/inventor/abdelhak-bensaoula?page=1

[7] Nacer Badi, Abdelhak Bensaoula. High Temperature Boron Oxynitride Capacitor/ Publication number: 20100157509, Publication date: June 24, 2010. http://patents.justia.com/inventor/abdelhak-bensaoula

[8]N. Badi, A. Bensaoula, A.V. Simakin, and G.A. Shafeev (2009) Nano-Engineered Dielectrics for Energy Storage Solutions. NSTI-Nanotech 2009,  978-1-4398-1783-4  Vol.  2, 2009, www.nsti.org/publications/Nanotech/2009/pdf/647.pdf

[9] David Starikov (Principal Investigator), Nacer Badi (Former Principal Investigator) NSF SBIR Phase II: Metamaterials for Giant Dielectrics and Energy Storage Solutions. Award Abstract #1026825, February 22, 2013. https://www.nsf.gov/awardsearch/showAward?AWD_ID=1026825

[10] R. Bikky, N. Badi, and A. Bensaoula  Effective Medium Theory of Nanodielectrics for Embedded Energy Storage Capacitors. Excerpt from the Proceedings of the COMSOL Conference 2010 Boston. http://www.imapsource.org/doi/pdf/10.4071/1551-4897-1.4.217