Nanocapacitor
Nanocapacitors are electrical capacitors whose separable individual structures are smaller than 100 nm in structure . They are currently (2009) research projects in nanotechnology . Two completely different objectives are being pursued. One area of research deals with a large number of individual nanocapacitors, arranged in a field, which can be charged and discharged separately; these can serve as information stores, the state of charge of each capacitor representing part of the information. The second development deals with the production of an array of nanotubes that are electrically connected to one another so that a high-capacity overall capacitor results.
Ferroelectric nanocapacitor as information storage
The aim of the research project at the Max Planck Institute for Microstructure Physics , Halle (Saale) is a new type of non-volatile, quickly writable and erasable solid-state memory with a storage density of close to terabits per square inch made of ferroelectric , single-crystalline nanocapacitors . In the 2009 activity report of this institute, the research results on these ferroelectric nanocapacitors were published.
Ferroelectric materials with precisely known properties have long been used, for example, in medical ultrasound technology or in ceramic capacitors . These materials contain a permanent electrical dipole in all unit cells, the smallest building blocks of a crystal . It comes about through the shift between positively and negatively charged ions in the unit cell. With the help of an electrical voltage , the polarity of this permanent electrical dipole can be reversed specifically and very quickly in the order of magnitude of nanoseconds . The polarity of this switched dipole remains stable, which means that the memory signal is not lost.
One of the most suitable ferroelectric materials for this is lead zirconate titanate (PZT). In order to produce a regular field of millions or billions of capacitors from this ceramic, a 100 nm thin stencil was first made of aluminum oxide using a nanotechnical process , which has a corresponding number of holes, each of which is smaller than 100 nm.
The template produced in this way was then mechanically reinforced and placed on a platinum layer which was applied to a plate of magnesium oxide . This lower platinum layer forms one of the two electrodes of the nanocapacitor. The ceramic material PZT was then evaporated in a controlled manner by means of an evaporation process. The steam penetrates through the pores of the stencil and is deposited on the platinum base as a 30–50 nm thin single-crystal ceramic layer. Another platinum layer, onto which the surface created above is vapor-deposited, forms the second electrode of the capacitor. Finally, the template is mechanically detached from the structure created. The many columnar nanocapacitors with contact on both sides remain in the array given by the template.
With an array of capacitors with a diameter of 40 nm, the smallest nanocapacitors produced to date, storage densities of 176 Gb / inch 2 were achieved. The comparatively low voltage of less than 1 V is sufficient to switch the ferroelectric dipoles in the PZT. A corresponding electrical hysteresis curve was demonstrated with the help of a converted atomic force microscope , a piezoresponse atomic force microscope, which was able to measure the slight deformation that occurred at this low voltage due to the piezoelectric effect of the PZT ceramic.
The storage of information ( bit ) with a structure size of around 40 nm / bit is in competition with semiconductor storage technologies . In 2009, integrated circuits , DRAM memories and flash memories in 45 nm technology were already state of the art . A storage product that is competitive for this would have to offer added value and maintain this value in the face of the ongoing continuous downsizing of semiconductor structures (cf. Moore's law ) and thus the information density. In addition, new improvements are constantly being added, such as the development of NAND flash memories with 3 bits per cell or more ( TLC memory cell , see also MLC memory cell ). However, only the future can show whether the future of the nanocapacitor as information storage is already sealed.
Nanocapacitor as a high-capacity capacitor
The Maryland NanoCenter at the University of Maryland, USA , under the direction of Gary W. Rubloff, is researching many interconnected nanocapacitors as high-capacitance nanocapacitors .
With this current (2009) research in the field of nanotechnology, high-capacitance capacitors are to be manufactured, whose electrical storage capacity is significantly higher than that of conventional capacitors and which are able to quickly absorb electrical energy and also release it again. This could result in a speed gap when storing and discharging electrical power, e.g. B. in new applications in automotive electronics or in wind turbines, because both double-layer capacitors (DLC) and accumulators cannot be charged or discharged as quickly as desired. Electrolytic capacitors can be charged and discharged relatively quickly, but they are significantly larger than double-layer capacitors or batteries.
This new type of high-capacitance nanocapacitor is basically a plate capacitor whose electrical charge is stored on two opposing electrodes that are separated from one another by an electrically insulating dielectric . Its capacitance is proportional to the surface area of the electrodes and inversely proportional to their distance from one another. In addition, the relative permittivity of the dielectric determines the size of the capacitance.
It is made up of the anodically produced base material aluminum oxide (Al 2 O 3 ). An extremely regular structure of tiny hexagonal nanopores is then etched into this material in a self-organizing, self-limiting and self-arranging ( self-assembly , self-limiting reaction and self-alignment ) nanotechnology etching process . Countless pores, each with a diameter of around 50 nm, can be produced next to one another. The depth of these tubes can be varied with the thickness of the base material.
In a special process called atomic layer deposition (ALD), wafer-thin titanium nitride (TiN), a conductive material, is applied as the lower base electrode to the aluminum oxide structured in this way with nanopores . An electrically insulating layer made of aluminum oxide Al 2 O 3 , the dielectric of the nanocapacitor, is then applied to this conductive layer, and finally a conductive layer made of TiN, the upper electrode, is applied over it again. The result is an arrangement of three layers: metal, insulator and metal (MIM structure), which lines the aluminum oxide structured with nanopores right into the pores and forms the actual capacitor. The metallic layers that form the electrodes are then contacted with the contacts of the later capacitor.
The scanning electronic image of the inner structure of the nanocapacitor shows that the three layers that make up the capacitor are only about 25 nm thick inside the pores. The insulating layer, the dielectric, is involved with about 6 nm. With a dielectric strength of the aluminum oxide of 0.7 V / nm at room temperature, the nanocapacitor should be suitable for a nominal voltage of 3 V. This is confirmed by the test results, which showed a dielectric strength of (4.1 ± 1.9) V or (4.6 ± 1.1) V at room temperature. When used in vehicle electronics at the upper limit temperature of 125 ° C that is customary there, the dielectric strength will drop to around 2 V.
The nanocapacitor has a specific capacitance of about 10 µF / cm 2 with a pore depth of about 1 µm and about 100 µF / cm² for a pore depth of 10 µm. According to the Maryland NanoCenter, this means a significant increase in the specific capacity per construction volume compared to previously known high-capacity capacitor technologies. According to the Maryland NanoCenter, the values of the power density (up to approx. 10 6 W / kg) exceed those of the electrolytic capacitors and the values of the energy density (approx. 0.7 Wh / kg) approximate the values of double-layer capacitors.
The prototype of a nanocapacitor, which the Maryland scientists presented in March 2009, consists of several dot-shaped arrays (dot capacitors) on a wafer with a diameter of about 125 µm each and containing about 1 million pores. By interconnecting the arrays, a capacitor with the desired properties can then be achieved. Further research on the new nanocapacitors will, for example, deal with the enlargement of manufacturable arrays and the material of the dielectric. Materials with a higher dielectric constant than aluminum oxide could increase the capacitance of the capacitor even further.
These and the many practical questions such as B. the encapsulation of the capacitors and above all the price, which are still open in the room, still stand in the way of rapid use of the nanocapacitors.
literature
- Kevin Zhang: Embedded Memories for Nano-Scale VLSIs . 1st edition. Springer, 2009, ISBN 978-0-387-88496-7 .
Individual evidence
- ↑ a b Dietrich Hesse, Marin Han, Woo Lee, Andriy Lotnyk, Stephan Senz, Markus Andreas Schubert, Ionela Vrejoiu, Ulrich Gösele: Ferroelectric nanocapacitors. In: Yearbook 2009 - Max Planck Institute for Microstructure Physics. Max Planck Institute for Microstructure Physics, Halle (Saale), 2008, accessed on January 21, 2010 (activity report with interesting images of the manufacture and appearance of the nanocapacitors).
- ↑ a b Toshiba Makes Major Advances in NAND Flash Memory with 3-bit-per-cell 32nm generation and with 4-bit-per-cell 43nm technology . In: Toshiba , February 11, 2009. Retrieved June 21, 2019.
- ^ Parag Banerjee, Israel Perez, Laurent Henn-Lecordier, Sang Bok Lee, Gary W. Rubloff: Nanotubular metal-insulator-metal capacitor arrays for energy storage . In: Nature Nanotechnology . tape 4 , no. 5 , 2009, p. 292–296 , doi : 10.1038 / nnano.2009.37 .
- ↑ Katherine Bourzac: Tiny sandwiches for the big hunger for energy. In: Telepolis. April 20, 2009. Retrieved April 20, 2009 .
- ↑ Nano Center Improves Energy Storage Options. In Nanotechnology Now. March 23, 2009. Retrieved August 11, 2009.
- ↑ New Electrostatic Nanocapacitors Offer High Power and High Energy Density. In Green Car Congress. March 17, 2009. Retrieved August 11, 2009.