Benefits of adding plasma
Discuss about the Carbon Nanotube Desktop Reactor with Plasma.
Carbon nanotubes show enormous panorama in uses through many industries such as the electronics and radical compounds. Growing isolated carbon nanotube is usually required most of these uses. Chemical vapour deposition procedures are now a routine in growing of carbon nanotubes. Chemical vapour deposition can support the growth of dense medium of elevated carbon nanotubes though cannot generate vertical carbon nanotubes. The latest study suggests that including plasma to carbon vapour deposition procedure can significantly enhance the possibility of developing vertical Carbon nanotubes. (Bishop, 2013)
- It helps in decomposing reactant gases used in the production of nanotubes.
- Generates an electric field that helps in the vertical alignment of the carbon nanotubes.
- Injects additional energy.(Chopra, 2012)
Background research reveals four principal means of generating plasma for the plasma enhanced chemical vapour deposition systems: direct current, inductively coupled, RF triode and microwave. Plasma enhanced vapour deposition methods are mainly classified into three modules, and another associate module design: plasma coil, reaction chamber, electronics, controllers of operational condition, substrate holder and the heating assembly. Customer necessities and provisions are used in providing a guideline for the formulation of the initial concepts pertaining every module. (Huimin Liu, 2013)
The ultimate design fits into 0.457m by 0.457m base platter and height within 0.305m. Reaction chamber comprises three orthogonal tubes mutually intersecting at the midpoint. A quartz tube is fixed at the end of one its tubes. A plasma wind is done on the tube to aide ignition of the reactant gases as they enter reaction chamber. Full substrate holder, adjustable electrode and the heating process get packed in one tray that’s able to slide into or out of reaction chamber. The infrared sensor and the chamber are designed into a stand containing a linear bearing that’s suitable for the purpose opening and closing of the chamber. (Jiyang Fan, 2014)
The whole chamber comprises commercially available assembled components, with the electrode, system tray, heat sink, and system stand and substrate system. (Franssila, 2010)
Essentially, a reaction chamber is the flundamental element of all plasma enhanced chemical vapour deposition systems hence much effort is accorded to it in terms of design. It’s this chamber that houses the process of nanotube growth as well as plasma generation. It comprises of viewports, chamber walls, in and out reactant gas passage and an electrical feed. The rest of the modules depend on reaction chamber design. The internal chamber as a module hugely relies on reaction chamber’s interior geometry. Therefore, concepts needed to be employed for reaction chamber design first, before much effort was afforded the other modules. (Sharpe, 2009)A pumping system, infrared sensor and fittings are selected to enhance the process of making controllers of an operating condition. As for the supporting electronics and plasma coil, a matching network, power supply and coil design are developed. An electrode assembly is designed as an extra design work in the internal chamber assemblage.
Strategies for plasma enhanced chemical vapour deposition
- Local plasma, plasma is created right over the substrate through growing environment.
- Remote plasma, plasma is generated separately from the growing environment after which it’s made to flow to the substrate. (Zhang, 2012)The main drawback of remote plasma is that weaker electric field necessary for nanotubes vertical alignment for vertical alignment of the nanotubes is easily overcome by any independent electric field separately generated near the substrate. Thus majority design concepts employ remote plasma.(Popov, 2008)
Seven concepts as illustrated in the figures below were generated for this design with two of the catering for every type of the geometry chamber. All the concepts utilized both the local and plasma approaches. Materials used as stainless and quartz. Adjustable electrode stands out as a key design module here. The box section containing plasma system has antennae have the shape of a heating coil, located on the outer side of the chamber to aide ignition of the local plasma. The front section is open to facilitate substrate access. Remote plasma disjoins the electric field and power source. The section box containing inductive plasma helps create the remote plasma that moves in the chamber. One is required to open gas outlet end thereby providing for substrate access. The tube holding the remote plasma helps in adding that plasma to the chemical vapour deposition system with the oven as a heat source.
Direct current substrate bias and remotely inducted plasma the above tube design employs the use of use of one quartz tube through which plasma is created using a coil at one end. The electrodes placed on opposite end allow for voltage supply. Pyrex cross design from commercial Pyrex cross has two ends, one for sample loading and the other one for plasma generation. A cross design (six-way) can also be generated in a number of ways. A steel chamber and spherical Pyrex with local plasma are also designed with the hemispherical Pyrex that allows us to view substrate through nanotube growth.
Comprises of the automatic adjustment which employs the use of a direct current motor the two wheels responsible for moving the electrode up and down. The seal has to be made dynamic enough to enhance electrode movement at a constant pressure difference within the chamber and atmosphere. There is also manual adjustment which uses a wound rod with the electrode attached to it. Rotation of the rod ensures that the electrode moves up and down. (Vossen, 2017)
Laboratory research by most lab users who have tried different chemical vapour deposition setups helps us make a choice among the two leading concepts of design. The vast experience among laboratory users enables them to interact with most useful features that are essential in improving the system usability. Watching the lab user’s setup, grow the carbon nanotubes also enables us to understand the handling of the samples. Professional interaction with lab technicians doing plasma research helps greatly in making the decision on the remote and local plasma. Furthermore, they provide information involving the advantages of both sources of plasma as well as a valuable necessary consultation knowledge on a suitable method of generating plasma.
Strategies for plasma enhanced chemical vapour deposition
Remotely inductive plasma source allows easy decoupling of the plasma as well as controlling the electric field. It’s also essential in improving system adjustability. Besides, it is relatively cheaper compared to microwave plasma and provides for a cleaner operation (Ostrikov, 2007)
Therefore, exploring the option of six-arms cross provides for an enormous adjustment to be made in the coming times. (Vladimirov, 2005)Hence the choice of a cross design. With limited project time frame, buying the chamber parts the components helps in ensuring that the parts assembled are easily capable of holding the vacuum as well as save on time that could be used machining them. (Ramesh, 2009)
The final designing program is put in three sectional assemblies as seen the figure below. They include;
- Reaction chamber- quartz tube that holds plasma and the body.
- Internal chamber- entails heating mechanisms, the electrode and the substrate holding.
- Base and supports – entails chamber supports, internal tray support, IR sensor with its support and the overall base support for other bases. The whole setup is shown below;
In making a design decision for the chamber, the cross geometry is purchased as it’s more flexible and has a huge range of standard components available to select from in choosing this. All of its projections attain Klein Flanges standards set by ISO thus allows profoundly easy fit all together. The gas is streamed through a fitting outlined in the diagram below. It then travels the quartz tube that is attached to two connect fittings to allow for easy exchange of fresh tubes as well as the tubes with different lengths. This tube is then wound with a copper wire, then linked to a plasma-producing generator. Another quick connection is then provided, to connect quartz tube with the other left end in the six-way cross-chamber. The upper and back The upper and back projection in this chamber has a number of cordial glass avenues that enable us to view the internal features as the lower projection helps us achieve an optical access to the IR sensor. The front projection allows for an electrical feeding passage consisting of individual eight sealed pressure copper pins. Thereafter, the gas flows out through the standard KF projection that is attached to the steel hose connected to the vacuum pump.
This chamber performs the task of giving mutual support to every element used in carbon nanotube making process. The essential section of assembly encompasses a system tray, that is connected to electrical feed through projection by use of a set of screw collars and steel pins (dowel pins). These pins are then pressed to fit in the projection and slip to exactly fit into the system tray. The lower three pins holding electric feed into the system can be removed to help knock off any interference they may bring to the system tray. (Seshan, 2012)
Adjustable Electrode Sub module
There are two rods in the grooves laid tray system, whose main role is supporting the gathering of the substrate and keeping it isolated electrically from the chamber and the system tray. Substrate assemblage comprises a heating element that is resistive (silicon) and the heat sinks (two). The heating element provides support to the substrate. The heat sinks hold the heating element, as well as allowing current through it, enabling it to heat up the substrate in the long run. The second figure below gives an outlook of how a heat sink is constructed (Mattox, 2016).
To create the electric field, an electrode is used. It is supported by the quartz rods vertically set in the tray system. Silver tipped screw sets are then used to control the height. The electrode and the heat sinks are connected to one of the feed pins each separately. The remaining two pins will be used in the removable thermocouple for the purpose of IR sensor calibration. Lastly, the connection between the actual clamps, wires, is determined. (Popov, 2008)
The base plate is made of alluminium and also holds the chamber supports, which are mounted to it. The supports help in keeping a fixed height of the chamber. It is also the base support into which the IR sensor is screwed. The stand for IR sensor gives a provision for adjusting the sensor vertically for the purpose of proper calibration to an appropriate height. The support for the system tray keeps the internal structures with system tray at the desired height designed. This holder, system tray, helps in proper adjustment of the stand aligning it with the chamber through an efficient vertical motion it provides. This holder is designed to be able to slide on the rail thus allowing movement of system tray completely from the chamber, as shown in the figure below. Essentially, it’s useful in substrate loading and off-loading, in the growing of the carbon nanotubes, enhancing cleaning of the whole system tray through its complete removal and replacement of the heating elements.
- Support the projection with the substrate frame on the slot of the chamber and then fix the gas to the tube inlet thereafter fix the pump to the chamber outlet.
- Slide the projection with substrate into the chamber and link every electrical feed from the projection supporting substrate to the electrodes.
- Put the substrate into a holder and then slide it gently and finish all the electrical links through the projections by linking the direct current power supply with the feed through.
- Link the coil with marching network, marching network and power source then the controller of the automatic network marching.
- Ensure all viewports and projections are in right positions.(Seshan, 2012)
- Induce a low pressure in the chamber by introducing inert gas like helium and open the gas flow while ensuring a desired rate of flow.
- Now put on the power source maintaining a desirable power level using an option of forwarding power in the generator, then wait until the generation of plasma in quartz tube is done, ensuring that the impedance source of 50 Ohms equals’ plasma impedance.
- After carbon nanotube reaches the desirable levels, put off the power source, vacuum and flow rates.
- Give it some minutes before embarking on dismounting of substrate stand and the flange in order to gain substrate access.
- Detach all the electrical connections carefully then take out the substrate. In case a direct current source is used instead, for power generation, then the electrode would have to be removed before getting an access to the substrate.(Popov, 2008)
In ensuring that the project meets engineering specifications, a number of required experiments have to be conducted (Etching, 2008). All of the mandatory tests to be performed are provided in the figure below, in an order of low priority in their undertaking. To avoid help avoid failure in some parts, wall temperatures and pressure of the chamber are checked and controlled. A calibrated IR sensor is used in measuring temperature, whose analysis helps determine if the required conditions are achieved, and the pressure measured by a pressure gauge.
Discussions and Observations
Plasma creation is visually seen through its ignition in the clear quartz tube. A confirmation of the growth of carbon nanotubes is achieved through the viewing of certain black growth occurring on the wafer or scanning by electronic microscope. Finally, SEM will go on to prove that this system has the capacity to produce isolated nanotubes vertically. (Franssila, 2010)
During the process of the undertaking of the project work, there is a certain foreseeable risk level that basically needs to be factored in the process of project work plan. It's therefore essential that we formulate the necessary measures to counteract these risks involved in the implementation process. Proper handling of such issues greatly helps in making the project within the stipulated timelines. The table below gives a summary of anticipated risks, problems as well as associated countermeasures (Vossen, 2017)
Evaluation of the above design concepts desires that a rating against the five critical criteria (adjustability, substrate access, fabrication ease, cost and size) be done to help come up with the best two options. System adjustability helps in increasing the rate of use of the system while at the same time reducing the ownership cost of the customer. Substrate accessibility enhances reduction in time between experiments as well as sample swapping between experiments. Easy fabrication enhances efficient assembling of the system thus meeting customer requirements profoundly. Cost as critical criteria helps keep in check project budget level reasonably well for the system. Desktop size then is critical to suit customer requirement. Every design is rated to ascertain its strength as well as a weakness with the best two options being a remote plasma box with six-way steel cross. (Ostrikov, 2007)
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