Patent Application
20150138692
This invention relates to coated structured surfaces, such as coated carbon nanotubes (CNTs) and an array of coated carbon nanotubes.
Many different electrically powered objects such as cars, computers, mobile phones, drills and hand held vacuum cleaners require a portable power source, and there is a drive to provide smaller, lighter and longer lasting portable power sources. Traditionally, portable power sources have tended to be provided by batteries. However, more recently other types of power source have been investigated. One such alternative is capacitors and more specifically supercapacitors.
Capacitors comprise two electric conductors separated by an insulator or dielectric. When a voltage is applied across the conductors, an electric field is generated across the dielectric and energy is stored in this electrical field. The stored energy can potentially be used as a power source. A capacitor can be recharged in a similar manner to rechargeable batteries. Conventional batteries have electric energy stored in a chemical form and the rate at which the battery can be charged or discharged depends on the rate at which the chemical reaction can occur. Dielectric capacitors do not depend on chemical reaction kinetics, and are orders of magnitude faster in terms of charging or discharging the stored electric charge as reflected in the high power densities. In addition, dielectric capacitors have life cycles which are much longer than those of batteries. However, conventional dielectric capacitors do not store enough charge compared to batteries, and therefore have a much lower energy density. A supercapacitor has a higher energy density than a capacitor and thus can store more energy per unit volume. To be a viable alternative to conventional rechargeable batteries, capacitors must have a similar or greater energy density than rechargeable batteries, have a similar cost to the consumer, and be similar in terms of weight and size. These are the technical problems that need to be overcome.
Electrophoresis is the motion of dispersed particles in a solvent under the influence of an electric field. This phenomenon is utilised in electrophoretic deposition (EPD) to coat a substrate with charged particles. EPD has been used to deposit coatings onto planer substrates, as described, for example, in the following publications: Fabrication of Ferroelectric BaTiO3 Films by Electrophoretic Deposition Jpn. J. Appl. Phys. 32 (1993) pp. 4182-4185 by Soichiro Okamura, Takeyo Tsukamoto and Nobuyuki Koura; and Preparation of a Monodispersed Suspension of Barium Titanate Nanoparticles and Electrophoretic Deposition of Thin Films. Journal of the American Ceramic Society, 87: 1578-1581(2004), doi: 10.1111/j.1551-2916.2004.01578.x by 2. Li, J., Wu, Y. J., Tanaka, H., Yamamoto, T. and Kuwabara, M; and Low-temperature synthesis of barium titanate thin films by nanoparticles electrophoretic deposition, JOURNAL OF ELECTROCERAMICS Volume 21, Numbers 1-4, 189-192, DOI: 10.1007/s10832-007-9106-6 by Yong Jun Wu, Juan Li, Tomomi Koga and Makoto Kuwabara,
According to a first aspect, the invention provides a method of coating a structured surface comprising the steps of:
The inventors have established that the EPD process is advantageous for use with structured surfaces that exhibit metallic behaviours as unlike other techniques such as spin coating and dip coating, EPD has been found to produce a conformal coating on micro and nano structured substrates.
Preferably, the structured surface comprises one or more carbon nanotubes. As carbon exhibits metallic behaviours, it can be used as a substrate for EPD.
Preferably, the carbon nanotubes are formed as an array of carbon nanotubes. This array may be a regular or a random array. It is preferred that chemical vapour deposition (CVD) is used to produce the CNTs; a D.C. plasma enhanced CVD growth chamber may be used to produce oriented nanotubes.
For the production of a regular array of CNTs, a substrate may be lithographically prepared to promote the growth of the CNTs only in specified positions. One preferred growth process consists of four stages:
In one example, the total pressure was maintained at 3.8 mbar and the depositions were carried out for 10 min in a stable discharge.
In a preferred embodiment, the structured surface comprises a random array of structures, preferably CNTs. Such a random array is also known as supergrowth and has significantly higher growth rate than a regular array. Preferably, the spacing to length ratio of the structures is a maximum of 1:30.
For supergrowth or random CNTs, a preferred growth process is as follows:
The CNT growth stage preferably has a duration which is no greater than 10 minutes, preferably between 1 and 10 minutes, even more preferably between 1 and 3 minutes. The aluminium layer is a barrier layer, and is used to form a thin alumina layer during the annealing process step. This thin oxide layer assists in forming iron nano-islands to grow CNTs in a high density. The substrate may be any conductive substrate. Preferably, the substrate is a copper or a silicon substrate. Alternatively, the substrate may be a graphite substrate.
In a preferred embodiment, the coating material is a dielectric material. Preferably the coating material is barium titanate (BaTiO3). Preferably, the particle size of the barium titanate is in the range of 70-150 nm More preferably, the barium titanate nanoparticles are 5-20 nm in diameter.
In one embodiment, the nanoparticles are agitated ultrasonically prior to being deposited onto the structured surface. This ultrasonic agitation shatters the nanoparticles into smaller particles, providing better coverage or a more conformal coating of the structured surface.
Advantageously, the material used in the second coating has properties which are complimentary to the first coating material. The second material provides a composite coating ensuring that the structured surface is completely coated. It is advantageous to have a complete coating as this stops any direct interaction between the structured surface and an external environment, for example in the case where the structured surface is an electrode of a capacitor, and so where direct interaction of the two electrodes would cause leakage of charge.
Preferably, the second coating material is a dielectric or high k metal oxide coating such as hafnium oxide, titanium dioxide, barium titanate and barium strontium titanate. Such coatings can be produced by various methods including but not limited to conformal atomic layer deposition (ALD), plasma enhanced ALD (PEALD), physical vapour deposition (PVD), pulsed laser deposition (PLD), metal organic chemical vapour deposition (MOCVD), plasma enhanced chemical vapour deposition (PECVD) and sputter coating.
In addition various polymer materials having relatively high K values can be used to form the dielectric, such as cyanoresins (CR-S), polyvinylidene fluoride-based polymers such as Pvdf:Trfe, or PVDF:TrFE:CFE, which can be spin coated onto the BTO coated CNTs. Self assembled monolayer coatings of phosphonic acids can also function as an additional coating to further reduce the leakage current.
The ALD process may comprise a plurality of deposition cycles, with each deposition cycle comprising the steps of (i) introducing a precursor to a process chamber, (ii) purging the process chamber using a purge gas, (iii) introducing an oxygen source as a second precursor to the process chamber, and (iv) purging the process chamber using the purge gas. The oxygen source may be one of oxygen and ozone. The purge gas may be argon, nitrogen or helium. To deposit hafnium oxide, an alkylamino hafnium compound precursor may be used. To deposit titanium dioxide, a titanium isopropoxide precursor may be used. Each deposition cycle is preferably performed with the substrate at the same temperature, which is preferably in the range from 200 to 300° C., for example 250° C. Each deposition step preferably comprises at least 100 deposition cycles. For example, an ALD deposition may comprise 200 to 400 deposition cycles to produce a hafnium oxide coating having a thickness in the range from 25 to 50 nm Where the deposition cycle is a plasma enhanced deposition cycle, step (iii) above preferably also includes striking a plasma, for example from argon or from a mixture of argon and one or more other gases, such as nitrogen, oxygen and hydrogen, before the oxidizing precursor is supplied to the chamber.
It is preferred that the dielectric coating is produced in a two step ALD process, whereby a first layer of the coating is deposited, followed by a pause in the deposition process and then a second layer of the second coating is deposited. This two step coating is applicable to both plasma only and combined plasma and thermal ALD coating methods. The pause is a break or delay in the deposition process which has been found advantageous to certain properties of the material deposited on the substrate. The delay preferably has a duration of at least one minute. The delay is preferably introduced to the deposition by supplying a purge gas to a process chamber in which the substrate is located for a period of time of at least one minute between the first deposition step and the second deposition step. Each deposition step preferably comprises a plurality of consecutive deposition cycles. Each of the deposition steps preferably comprise at least fifty deposition cycles, and at least one of the deposition steps may comprise at least one hundred deposition cycles. In one example, each of the deposition steps comprises two hundred consecutive deposition cycles. The duration of the delay between the deposition steps is preferably longer than the duration of each deposition cycle. The duration of each deposition cycle is preferably in the range from 40 to 50 seconds.
The delay between deposition steps may be provided by a prolonged duration of a period of time for which purge gas is supplied to the process chamber at the end of a selected one of the deposition cycles. This selected deposition cycle may occur towards the start of the deposition process, towards the end of the deposition cycle, or substantially midway through the deposition process.
According to a second aspect, the invention provides a method of manufacturing a capacitor having an electrode with a structured surface, comprising the steps of:
Preferably, the first dielectric material is barium titanate. It is preferred that the barium titanate particles are approximately 20 nm in diameter.
It is preferred that the second coating is formed using atomic layer deposition. Preferably, the second coating is hafnium oxide.
Alternatively, the second coating may be formed using physical layer deposition. IN this case, the second coating may be barium titanate.
Preferably, the second electrode is produced using evaporation of a conducting material for example aluminium or galinstan.
According to a third aspect the invention provides a capacitor comprising:
Preferably, the structured material is an array of CNTs. The array may be regular or random.
Preferably, the dielectric layer is formed using EPD; when the structured surface is coated with a dielectric material using EPD, this results in the production of a conformal coating. This provides a less leaky material, as the two electrodes do not come into direct contact. It is preferred that the dielectric layer is formed from barium titanate.
The dielectric layer comprises a first layer and a second layer. Preferably the first layer is barium titanate. It is preferred that the second layer is hafnium oxide.
To form a capacitor a second electrode is required. It is preferred that the second electrode is formed from a metal or intermetallic material such as, but not limited to aluminium, titanium nitride, ruthenium, and platinum which can be deposited onto the coated CNT using ALD for example. In addition, a liquid metal such as galinstan may be evaporated onto the structure.
The invention will now be described by example with reference to the accompanying drawings, of which:
a and 2b show images of barium titanate deposited by EPD onto CNTs;
a and 3b show TEM and TEM diffraction images of barium titanate particles;
a and 4b show synthesised and commercial barium titanate nanoparticle coatings;
A regular array of CNT's was grown by PECVD (plasma enhanced chemical vapour deposition) on an e-beam lithography patterned high conductivity p-Si substrate with a 25 mm2 area.
The solution 150 comprises a 1g/litre concentration of barium titanate, BaTiO3, (BTO) particles dissolved in water. The negative electrode 140 is a CNT array and when the power source 120 is switched on positively charged BTO particles are attached to the negative electrode and thereby coat the CNT array. The solutions of BTO had nanoparticles of size range 70-150 nm. The nanoparticles were dispersed in the solution for 6 hours by ultrasonication using a tip sonicator at 200 to 250 W to produce a stable suspension which was transferred to an electrophoretic cell with electrodes 2 cm apart.
a shows a BTO coating on a regular CNT array formed by carrying out an electrodeposition process at 10V for 5 seconds and
Unlike when BTO is deposited using EPD on flat substrates where film thickness scales linearly with concentration and dilution results in denser films, when structured substrates are used the growth rate depends on DC bias and concentration of the suspension.
Although EPD provides a conformal coating, the size of the BTO particles results in a non-continuous coating. One partial solution to this is to use smaller particles. Two different techniques were used to produce smaller particles.
In a first technique BTO nanoparticles were prepared solvothermally or hydrothermally using barium hydroxide octahydrate and titanium (IV) tetraisopropoxide. The resulting nanoparticles were 5-20 nm in diameter with cubic perovskite phase crystallinity. The reactants were as follows:
Ba(OH)2+8H2O+Ti{OCH(CH3)2}4(Titanium isopropoxide)+Ethanol (60 ml)
The solution was placed in a water bath at 50° C. for 4 hours under magnetic stirring. Then, the product of the reaction was washed with formic acid, ethanol, and finally de-ionised water and subsequently dried at 50° C. for 6 hours in vacuum.
In a second technique, commercially available 70-150 nm BTO nanoparticles (available from Sigma-Aldrich) which are generally spherical in shape were subjected to high power ultrasonication which caused shattering of the particles to approximately 20 nm in size (with a range of 4 nm-25 nm).
The larger particles were suspended in water using a tip sonicator at 200 W to 250 W for 6 to 12 hours. A tip sonicator provides more power per unit volume at the tip than an ultrasonic bath. This technique is usually carried out using an organic solvent to disperse the particles rather than water as water dissolves the particles. However, it is thought that particles dissolve in the water and then re-crystallise because of the high energy input at the tip of the tip sonicator to produce sharp fragments of BTO. There is natural circulation of the particles within the suspension due to the tip sonicator so a constant stream of material is provided near the tip. Once the sonication process was complete, the suspension was left for at least one hour to enable settling of the larger particles to the bottom of the suspension.
These nanoparticles were then coated onto CNTs using EPD.
However, the coated CNTs were still electrically leaky, and this is considered to be due to the coating not being continuous and, as the nanoparticles deposit much better on the nanotubes than on the silicon substrate, which creates a leakage path between the two electrodes. It is important for a capacitor to have a good, complete insulating layer otherwise stored charge will be lost over time. To mitigate this problem, a second coating material was provided. This second coating is preferably a material with a high K value i.e. high permittivity.
Examples of compounds which are suitable for use as the second coating material include, but is not limited to, high k metal oxide coatings such as hafnium oxide, titanium dioxide, barium titanate, and barium strontium titanate, which can be coated by various methods including but not limited to conformal atomic layer deposition (ALD), plasma enhanced ALD (PEALD), physical vapour deposition (PVD), pulsed laser deposition (PLD), metal organic chemical vapour deposition (MOCVD), plasma enhanced chemical vapour deposition (PECVD) and sputter coating. In addition various polymer materials having relatively high K values are available such as cyanoresins (CR-S), polyvinylidene fluoride based polymers like Pvdf:Trfe, PVDF:TrFE:CFE, which can be spin coated onto the BTO coated CNTs. Self assembled monolayer coatings of phosphonic acids can also function as an additional coating to further reduce the leakage current.
A PEALD process was conducted using a Cambridge Nanotech Fiji 200 plasma ALD system. The substrate was located in a process chamber of the ALD system which was evacuated to a pressure in the range from 0.3 to 0.5 mbar during the deposition process, and the substrate was held at a temperature of around 250° C. during the deposition process. Argon was selected as a purge gas, and was supplied to the chamber at a flow rate of 200 sccm for a period of at least 30 seconds prior to commencement of the first deposition cycle.
An example of a second coating is shown in
A preferred PEALD process to form a hafnium oxide coating comprises a series of deposition cycles. Each deposition cycle commences with a supply of a hafnium precursor to the deposition chamber. The hafnium precursor was tetrakis dimethyl amino hafnium (TDMAHf, Hf(N(CH3)2)4). The hafnium precursor was added to the purge gas for a period of 0.25 seconds. Following the introduction of the hafnium precursor to the chamber, the purge gas was supplied for a further 5 seconds to remove any excess hafnium precursor from the chamber. A plasma was then struck using the argon purge gas. The plasma power level was 300 W. The plasma was stabilised for a period of 5 seconds before oxygen was supplied to the plasma at a flow rate of 20 sccm for a duration of 20 seconds. The plasma power was switched off and the flow of oxygen stopped, and the argon purge gas was supplied for a further 5 seconds to remove any excess oxidizing precursor from the chamber, and to terminate the deposition cycle.
The deposition process was a discontinuous PEALD process, comprising a first deposition step, a second deposition step, and a delay between the first deposition step and the second deposition step. The first deposition step comprised 200 consecutive deposition cycles, again with substantially no delay between the end of one deposition cycle and the start of the next deposition cycle. The second deposition step comprised further 200 consecutive deposition cycles, again with substantially no delay between the end of one deposition cycle and the start of the next deposition cycle. The delay between the final deposition cycle of the first deposition step and the first deposition cycle of the second deposition step was in the range from 1 to 60 minutes. During the delay, the pressure in the chamber was maintained in the range from 0.3 to 0.5 mbar, the substrate was held at a temperature of around 250° C., and the argon purge gas was conveyed continuously to the chamber at 20 sccm. This delay between the deposition steps may also be considered to be an increase in the period of time during which purge gas is supplied to the chamber at the end of a selected deposition cycle. The thicknesses of coatings produced by both deposition processes were around 36 nm.
Titanium dioxide coatings where also deposited onto a BTO coated CNT.
Four titanium dioxide coatings were formed on respective silicon substrates, each using a different respective deposition process. The first deposition process was a standard PEALD process comprising 400 consecutive deposition cycles, with substantially no delay between the end of one deposition cycle and the start of the next deposition cycle, and the variation in dielectric constant of the resultant coating with voltage is indicated at 30 in
The second deposition process was a discontinuous PEALD process, comprising a first deposition step, a second deposition step, and a delay between the first deposition step and the second deposition step. The first deposition step comprised 200 consecutive deposition cycles, again with substantially no delay between the end of one deposition cycle and the start of the next deposition cycle. The second deposition step comprised further 200 consecutive deposition cycles, again with substantially no delay between the end of one deposition cycle and the start of the next deposition cycle. The delay between the final deposition cycle of the first deposition step and the first deposition cycle of the second deposition step was 10 minutes. During the delay, the pressure in the chamber was maintained in the range from 0.3 to 0.5 mbar, the substrate was held at a temperature of around 250° C., and the argon purge gas was conveyed to the chamber at 20 sccm. The variation in dielectric constant of the resultant coating with voltage is indicated at 40 in
The third deposition process was similar to the second deposition process, but with a delay of 30 minutes, and the variation in dielectric constant of the resultant coating with voltage is indicated at 50 in
At negative voltages the graphs for the discontinuous processes are very similar, and the dielectric constant is higher than the zero voltage level for the continuous deposition process. At positive voltage, the coating produced using the second deposition process had the highest dielectric constant.
The use of a second coating produces a coated material having a lower leakage current and lower capacitance.
The coated nanotubes can be used in a capacitor or as a three dimensional ferroelectric memory.
To form a capacitor, a second electrode is required. It is preferred that the second electrode is formed from a metal or intermetallic material such as, but not limited to, aluminium, titanium nitride, ruthenium, and platinum which can be deposited onto the coated CNT using ALD for example or evaporated using an Edwards vacuum evaporator. In addition, a liquid metal alloy such as galinstan may be evaporated onto the structure.
For example a metal-insulator-semiconductor (Al/HfO2/n-Si) capacitor structure was made by applying dots of aluminum on top of the hafnium oxide coated silicon substrate. The dots were 0.5 mm in diameter and were made by evaporation of aluminum. The four hafnium oxide-coated silicon substrates were formed using the four different deposition processes. A first hafnium oxide-coated silicon substrate was formed using a continuous process. A second hafnium oxide-coated silicon substrate was formed with a delay having a duration of 1 minute instead of 10 minutes. A third hafnium oxide-coated silicon substrate was formed with a delay having a duration of 30 minutes instead of 10 minutes. A fourth hafnium oxide-coated silicon substrate was formed with a delay having a duration of 60 minutes instead of 10 minutes. In all cases the delay occurred after 200 deposition cycles. The capacitance-voltage characteristics of the four coatings have very little hysteresis and the presence of the delay between the deposition steps provides an increase in the capacitance of the capacitor.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1207764.0 | May 2012 | GB | national |
This application is a national stage application under 35 USC 371 of International Application No. PCT/GB2013/051049, filed Apr. 25, 2013, which claims the priority of United Kingdom Application No. 1207764.0, filed May 3, 2012, the entire contents of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2013/051049 | 4/25/2013 | WO | 00 |
