Carbon Nanotube Desktop Reactor With Plasma Enriched Growth

Describe the Carbon Nanotube Desktop Reactor With Plasma Enriched Growth.

The project is focused towards generating carbon nanotubes which develop vertically and that are enabled by the principle of electromagnetic fields which makes the carbon nanotubes get aligned and at the same time energizes the reaction process.  

Methods of generating plasma chemical vapor deposition system

 There are four methods that can be used in developing the plasma chemical vapor deposition system. These methods include

• Microwave
• Radiofrequency triode
• Direct current
• Inductively Coupled  

Below are the figures showing the various methodologies.

The engineering specifications and the customer requirements help in developing the design of the carbon nanotube. Various meetings were held which comprise of investors and sponsors to develop the customer requirements which were then used in coming up with the engineering specifications. Besides, various consultations together with benchmarking were done in order to develop the engineering specifications. A quality function deployment was used in analyzing the specifications.  The tool was the one which identified the very significant design aspects such as  (Chopra, 2012)

• An adjustable electrode cap which varies the conditions of the electric field
• Sizable System size which can easily fit into the desktop
• Operating condition controller

From the research done, it was established that plasma chemical vapour deposition system can be divided into various parts. These parts include

• Reaction chamber – these reaction chambers comprised of the central module which housed the internal chamber assembly.
• Plasma coil and electronics – this provides support to the reaction chamber module
• Operating condition controllers – this also supports the reaction chamber module

The main component of the plasma chemical vapor deposition system is the reaction chamber and plays a significant role while developing the design modules.it helps in generating the plasma as well as giving grounds for the development of the nanotubes. The various parts of the reaction chamber include the viewports, reactant gas, an electrical feedthrough, chamber walls, the outlet and inlet. Plasma generation is aided by the operating conditioners, plasma coil and the supporting electronics.

The rest of the design modules and the submodules depends on the design of the reaction chamber. Hence, it was very imperial to first come up with the design of the reaction chamber before proceeding to design the other modules. Once the reaction chamber had been completely designed, the other modules and sub-modules were designed such as the plasma coil and the attached electronics it comprises of the coil design and the selection of the suitable power supply system that would best suit the system (Ebbesen, 2010).

Besides, the selection of the sensors and the fittings were also done alongside.  Hence, after the selection of all the above system components, a preliminary design was developed which was eventually converted into o the conceptual design as would be demonstrated below.  

With references to the research, there were two techniques which were established in developing the design of the plasma chemical deposition. This method includes the remote and the local techniques. For the local plasma design systems, the plasma directly gets generated over the substrate in the provided environment. Whereas the remote plasma design system the substrate gets developed on a separate substrate. Below are some of the comparison of the two techniques

	Local plasma	Remote plasma
Benefits	The electric field is developed on the substrate	There is no development o the  electric field over the substrate
Limitation	The electric field can end up destroying the nanotubes	There is a weak electric field generated on the substrate

Methods of generating plasma chemical vapor deposition system

From the above comparisons, it can be seen that for the remote design, there is the weaker electric field generated on the substrate, hence it is the preferred technique.

 Below are some of the system component concepts which were developed.

• Inductive plasma box

this box uses an antenna which is wrapped in a heating coil. The coil is meant to locally excite the plasma gas and it is located outside the chamber with its front open left open for the purpose of lessening the substrate access.

• Box having inductive plasma with dc substrate bias

This box is able to move into the chamber. It provides an allowance for the disconnecting the source of the plasma and the electric field thereby making it easier for the user to open the box and aces the substrate. Below is the diagram showing the box (Ebbesen, 2010) 

• Tube with remote inductive plasma and oven heating

This tube supplements the system with plasma and the heating source that is used is the oven. There is a coil which is electrically biased and is wrapped around the tube inlet. Below is the diagram of the tube

• Tube with remote inductive plasma and DC substrate bias

It uses one quartz tube that generates plasma one just one end of the coil.  At the end, the caps are the electrodes which are attached for application of a voltage (Etching, 2008).

• Pyrex cross with remote inductive plasma and DC substrate bias

This design is generally constructed from Pyrex which is commercially available.  One side of the arm helps in generating the plasma while the other arms help in coming up with the loading sample. Below is the design of the tube

• Pyrex or steel six-way cross with remote inductive plasma and DC substrate bias  

 This tube comprises of a six-way cross design that is modelled by different means. There are three holes which are attached to the stainless steel and they have adjustable configurations.

The sub-modules which were generated are the internal assembly chamber and the adjustable electrode (Franssila, 2010).

• Adjustable electrode

This sub-module helps in developing the preliminary concepts of the design. It comprises of an automatic height adjustment concept that applies a direct current motor in order to displace the wheel as well as maintaining the pressure difference in the outside atmosphere and the chamber.  Shon below is a preliminary concept of the automatically adjustable electrode.

It applies the idea of the direct current in order to change the position of the wheel.

• Manually adjustable electrode

 For the manually adjustable electrode, a threaded rod is applied which is connected to the rotating rod that in turn moves upward and downwards. There is a sealing in form of a cap which is used prevent the chamber from being affected by the outward atmosphere (Franssila, 2010).  

• Internal chamber assembly

Engineering specifications and the Customer requirements

This part is also very crucial in the design of the submodules. It also helped in generating the conceptual design.

Conceptual screening and selection

In order to effectively evaluate the design concepts, they were rated against five such that it will be easier to select out the best two.These design concepts which were utilized includes fabrication, substrate access, cost, size and finally the adjustability.  In order to maintain the budget of the design project, the cost plays a significant role. Since the system needs to be assembled, the fabrication factor comes in, which must be in line with the requirements of the customer. The ownership cost is minimized by increasing the number of ways of the system by the adjustability factor (Franssila, 2010).

Additionally, easy ace to the substrate is as well important for the purposes of swapping out the samples. This kind of methodological criteria helps in clearly understanding the various strengths and weakness of the design system. The table below shows the results of the rating.

 A consultation with the various professional doctors was done for purposes of deciding on the major design concepts and from the advice received, many design concepts should be applied in order to increase the accuracy chances. In order to choose between the remote and the local, a consultation with 2 professors helps in choosing the best method with reference to their comparison in terms of the advantages of each method (Liu, 2013).

The method that is appropriate for the design of the project is by the use of a remote plasma source. This is because this technique will greatly aid in disconnecting the electric field and the plasma as well as enhancing the adjustability of the system. In addition, having a box shape design will promote easy access to the substrate. The diagram below shows the complete design of the system with its main three parts. It comprises of the main reaction chamber body and the quartz tube.  Thereafter, there is the internal chamber assembly which is composed of the holder of the substrate and the various heating components. Finally, there is the support and the base which is also composed of the infrared sensor, the chamber support, the internal system as well as the base system.

In the design of the chamber, a 2inch diameter geometry is selected since it has a wide range of components. The flanges are chosen such that they comply with the Klein flanges standards. The fitting on the left side of the chamber will permit the entry of the gas. The tube is in turn connected with a fitting which allows easy changing of the tubes. Thereafter, the tube gets wrapped using copper material which is attached to the radio frequency generator. This helps in generating the plasma. The other connection helps in attaching the quarts to the left side of the fitting tube. At the top and bottom, there will be the presence of Kodiak glass which aids in the viewing of the components which are located in the chamber (Markku et al, 2009).

Modules and design strategies

Besides, the Kodiak glass will easily allow for the access of the infrared sensor located at the chamber. Upon getting out of the flange, the gas is channeled to the vacuum pump.  Below is the diagram representing this?

The design of the internal chamber is generally meant for providing safer and efficient environment both for the internal components and the growing carbon nanotubes. The main frame of the internal design is the system tray that is connected to the flange to prevent cases of interferences with the flange, the bottom part of the feedthrough pins are clipped. There is the presence of quartz rods which supports the assembly and in turn electrically maintain the chamber and the system tray. The substrate is sputtered by the silicon resistive g heating element and the heat sink which composes the substrate assembly (Mattox, 2014).

Apart from providing support, the heat sinks also allow for the flow of current by allowing the heating element in position. There is the presence of a plate with two holes which are threaded. These holes are purposefully meant for screwing up the thumb screw. To generate an electric field, two quartz rods together with an electrode will be used whereby one of the metallic feedthrough pins will get connected to the electrode while the other remaining will be used for the purposes of calibrating the infrared sensor.  Below is how the connection looks like

Generally, the base helps in providing support to the whole system and it is generally made of aluminum plate. Support is crucial for ensuring that the system is fixed and in a good position. Besides, it gives the infrared sensor an easy time while calibrating. The figure below demonstrates how the support looks like (Mattox, 2016)

View of internal subassembly completely removed from chamber by sliding on rail

The quartz tube is wrapped using an inductive coil which in turn is connected to the shunt capacitor. The shunt capacitor also is connected to the radio frequency power generator and a controller. The plasma acts as the load while the source acts as the generator. Hence, there is an automatic matching which connects the load and the generator. In order to minimize the disturbances that may be caused by the outside surrounding of the design system, it is wrapped using a copper mesh which also acts as the faraday (Nanomaterials, 2011).

Below is the representation of the electrical set up

Design methodologies

The table below provides the approximated cost for the whole project. As can be seen, only the feedthrough and the flange are a bit cheap since they are easily weldable. Despite this, the steel has a shorter lead time of approximately a week. Besides, the most expensive components of the radio frequency generator which provides the matching network with the power of 500W.  From the analysis, it can be seen that only 300 w is necessary for this project. But then, the 500 w is significant such that it provides a wider range of operation and thereby making the system more flexible (Ostrikov, 2008).  

The matching network generally plays a vital role in transforming the plasma impedance to that of the radio frequency generator thereby eliminating cases of buffering in the generator. The matching of the load should be carefully done else some of the power will get reflected back into the generator .besides, to meet the demand and preferences if the customer, two pumps are required that rubs at a pressure of 0.6 hp.

The plasma modelling is c done with the aim of determining the input parameters such as pressure and power.  From the modelling results generated, the current power and the pressure were also determined. Besides, suggestions on the various variations in temperature as well as the other varying geometries were also provided.  The through-put formula also assisted in determining the pressure and the flow rate of the system (Popov, 2008). 

From the CES EDU-PACK software, the dimensions of the materials that were calculated and given out. There was a restriction on the use stainless steel and quartz only. This was generally meant to minimize the reaction of the plasma with the walls of the tube of the inner chamber as well as the holder to the substrate. The table below shows the materials and dimensions elected

Nevertheless, the stainless steel and quartz also have a higher chemical stability and service temperatures thereby becoming ideal for the project. Their outgassing rates are also low thereby substantially they do not add any material to the composition of the plasma (Vossen, 2017).

Potential failures in the system, effects of such failures and the required action to be adopted to prevent the failures were identified through the failure mode and effect analysis (FMEA). The FMEA was conducted through the use of design Safe software and the obtained reports as illustrated in Appendix J. the key areas generated from wiring, routes for evacuating gas, availability of reactant gases and the mechanical dimension of design. A list and explanation of failure modes and safety concerns of the design are provided in the table below.

System concepts

As evident from the table, it is observable that a good number of safety concerns generates from tubing leaks connections and faulty wiring involving electrical connections.  As a result of an inability to directly address these concerns in the design, with an aim of preventing the above concerns, it is in order to regularly check the tubing and electrical connections and consistent leak checking. In addition, another key concern relates to the temperatures whereby the maximum temperature arrived at by the system was hardly accurately predetermined by the modelling system.   It was expected of the system to attain temperatures higher than the temperature reached by the modelling thus making it a key concern for the quartz tube, chamber and gas tubing O-rings connection.  In solving this issue, it was decided to implement a provisional solution of putting fans of muffin along the coil and tube length.  This procedure was considered easy, simple and less expensive with high effectiveness in reducing the temperature of the system. Despite the temporary solution of the concern, a more permanent solution of implementing the use of water jacket or cooling the quartz tube and the coil tube by use of a cooling fin would be considered the best option as a long-term remedy (Vossen, 2017).

The above safety concerns formed the main reason for performing FMEA analysis and a report on failure modes and risk levels were developed together with applicable considerations aimed at reducing the level of risk with respect to each.

 The effects of the design on the environment were analyzed through use of Simapro software. The below figure illustrates the plot for the total raw, water, air, emission of soil and waste for 2300g of 304 L stainless steel and 200g of glass. The maximum raw emission is led by stainless steel at roughly 8000g seconded by the glass at 4000g. In comparison to the total raw emission, other emissions are of limited impact (Sharpe, 2009).

The diagram below shows stainless steel impacts and quartz manufacturing to the content of the mineral, use of land, acidity, eco-toxicity, the ozone layer, and change in climate, radiation organics, and inorganics. Most of the effects are brought by the stainless steel as can be observed.  In order to reduce environmental impacts, a different material which may pose a challenge since stainless steel is easily available needs to be used for the chamber. Nevertheless, stainless steel is less expensive and best suitable for non-reactive conditions and machinability and eventually, it is capable of enduring plasma making it, in this case, most suitable and desirable.`

Sub-modules

Manufacturing stainless steel and quartz possess little effects on human health in comparison to natural resources as shown in the two figures below. The diagrams are in relation to the raw emission figures for both t5he quartz and stainless steel (Seshan, 2012).

This section illustrates the procedures and steps carried out in manufacturing the system of PECVD. It categorically explains the components that are professionally machined via the water jet and upon which purchase of the components are based on. In addition, it involves the process of assembly applied in building the concluding prototype (Ramesh, 2009).   

 As a result of constraints in time and insufficient machineability by the project team, there exist three distinct components that are professionally machined, namely:

• Two heat sinks
• System tray

These components are shown in the figure below with appendix K illustrating drawings for engineering illustrating the outsourced components.

 A water jet machine is used with an aim of making all the components to manufacture in-house in in order to decrease time and cost of machining. Below is an illustration of inner components of a chamber that are cut through the use of a water jet.

The 4 stands, guide block plate, base plate and internal subassembly connector plate are made with the water jet. Two holes are drilled into the stands intended to attach them to the base plate. Guide holes in the base plate are made by the water jet. The water jet cuts the guide block plate and the holes drilled out and then tapped to attach the stand to the guide block. The water jet is again used to cut the connector plate. The diagram below illustrates the components of the stand that are manufactured by the water jet (Pierson, 2012).

Maintenance of the vacuum seal is logically very demanding and too costly to have machined making many components in the system to be critical and more important. Most components such as rails and guide block, electric components and fasteners are purchased.

A good number of components of the chamber if not all are purchased from Kurt J. Lesker which is a vacuum science company which majors in the sales of this components. All these components easily assemble and properly fits together. The figure below shows one of the key components purchased from the said company.

The two dowels that are to be pressed to fit through the flange of the electric feedthrough forms the most complicated and difficult process of manufacturing for the chamber. It is thus a very challenging step of the process of manufacturing hence a special and precise need of implementation in order to enable the system tray to attach itself to the flange similarly holding up of the flange to the tray. The design categorically calls for the 4 rods of quartz and a single tube of quartz.  These components are purchased from the G. Finkenbeiner Inc (Ostrikov, 2007).

Conceptual screening and selection

The stand of flange consists of a post holder, base adopter, post and forks for clamping that are specifically purchased or bought from Thor Labs. Besides these components, the V mount and a right-angled plate that are majorly used in holding the sensor of IR are also bought from the same firm, Thor Lab. The diagram below shows the purchased components from the Thor Labs.

The IR sensor is sold by the Exergen Company limited hence the purchased from the mentioned company. The set screws, pins of dowel, miscellaneous screws and collars of the shaft are directly purchased from the McMaster Carr.  Besides these components, guide block and rail are also purchased from the McMaster Carr.  A generator of RF and matching network are either bought or purchased from the PTB sales company or borrowed from the University of Michigan lab specifically the Nanofabrication. A copper wire of a quarter diameter inch is bought with an aim of making the coiled plasma despite the vendor not to have been decided yet (Nicholson, et al., 2008-12).

This section explains in details the manner in which all the components are assembled in order to produce the very last prototype. Below is a flowchart illustrating the key steps and procedures applied in assembling the prototype as well as the completion general order.

The chamber assembly consists of all the components of Kurt J. Lesker together with the tube of quartz. Initially, before the assembling of the chamber, the dowels must be press fit through the electrical feedthrough flange. On the completion of this, all the used flanges are easily attached to the 6-way cross through the use of clamps. A quick connect on each end which is attached by use of the same clamps type similar to chamber flanges are used to attach the quartz tubing.

The system tray assembly comprises of the system tray itself, electrode, four quartz rods, heat sinks, silicon wafer, four plates, two shaft collars, and two set screws with tips of silver. On the system tray, two quartz rods are laid then two heat sinks are placed on top. Placed in between the two heat sinks and the plate is the silicon wafer. This is done so that the silicon wafer can be kept safe and secure. Two holes are then made on the tray where the two smaller quartz rods are erected. Two screw sets are used to secure the electrode which is slid onto the two standing quartz rods. Then using two shaft collars the tray is attached to a press-fit dowel pin. The system tray assembly picture is as shown in figure 13 (Fan, 2014).

Discussions

This part comprises of the following parts, that is, the rail, stand, connector plate, two shaft collars, a guide block, and a guide block plate. Using two shaft collars and a connector plate, an electrical feedthrough flange is attached to the system tray stand. The guide block plate connects the guide block and the stand which is connected to the guide block flange.

Attached to the base plate using bolts will be the rail and the stands. In addition, an IR sensor mount will be connected to a right angle bracket, the attached to the base plate using bolts.

INITIAL SET UP

• Attach the substrate holder flange on to the chamber. Take caution not to place the substrate inside the chamber.
• Attach the quartz tube on to the chamber followed by sliding in the inductive coil around the tube. To the inlet of the quartz tube, a gas tubing is connected and the outlet of the chamber is connected to the pump.
• Inside the chamber, the flange containing the substrate holder is made to slide out.
• All the electrical feed-throughs from the electrodes, flange holder, and the substrate holder are connected.
• Gently slide in the flange into the chamber after mounting the substrate to the substrate holder (Vossen, 2017).
• Connect the electrical feed-throughs to a DC power supply. This completes the electrical connections in the flange.
• To the matching network connect the coil, to the RF power generator and the automatic matching network regulator connect the matching network.
• Countercheck that all the viewports and the flanges are in place then completely seal the chamber.

PROCESS SET UP

• Pass an inert gas such as helium in the chamber to reduce the pressure of the chamber to low levels.
• Ensure the gas flow rates are turned on to the desired levels.
• To the desired level of power turn on the RF generator. This is done by selecting the forward power option in the generator, 5then wait till plasma is produced in the quartz tube.
• Countercheck that the source impedance of say 50 ohms is matching the complex load impedance of plasma by tuning, for the maximum power transfer from the source to the load (Vladimirov, 2005).
• Turn off the generator followed by the flow rates than the vacuum at intervals, thereafter the growth process of the CNT reaches its desired level or its final stage.
• Wait for a little then dismount the substrate holder and the flange. This gives access to the substrate.
• Then to remove the substrate, keenly and with care disconnect all the electrical connections.
• By using a DC source instead of an RF generator, then an extra step of removing the electrode before the substrate would be recommended.

CLEANING THE QUARTZ TUBE AND THE CHAMBER.

• Countercheck whether all vacuum the pumps, gas flow rates and the power are turned off before doing anything.
• Dismount the quartz tube from any hosing connection.
• Remove the coil (Mattox, 2014)
• With care, dismount the quartz tube. This done by disconnecting the tube and the chamber.
• Slowly slide off the flange that bears the substrate holder in the chamber.
• Disconnect and remove any other connections to the chamber for the accessing and cleaning the inner chamber.

CHANGING ELECTRODES AND THE QUARTZ TUBES.

• With care, remove the flange carrying the substrate holder by it sliding off.
• Make lose the set screws holding the electrode on the vertical quartz tubes then gently slide it off.
• Disconnect the tubes from the substrate holder for cleaning or replacement purposes.

For the meeting of certain engineering specifications, some experiments must be done. The figure 36 below lists specifications in the order in which they will be tested. To begin with, test the pressure and temperature controls to check if the chamber pressures and the temperatures of the walls are too high. To measure temperature an IR sensor with calibrations is used, while to measure pressure, a pressure gauge is used.

Analyzing these sensors will help in telling whether the desired conditions can be achieved. Consequently, the creation of the plasma must be met and this will be confirmed visually as we will be able to view an ignition of the plasma through the glass tube. Similarly, by seeing black growth on the wafer we will be able to tell that carbon nanotubes are growing. Besides, we can further confirm the growth of the nanotubes using a microscopy technology known as a scanning electron microscope. In addition, the scanning electron microscope will also help us to prove that the system can achieve vertical and isolated nanotubes.

To finalize the requested work within three months a given level of taking the risk was required. Countermeasures were established to ensure that none of the risk taken would lead to incomplete work. Nevertheless, these countermeasures were also established to help avoid any problem that would occur. The table below summarizes the risks or the countermeasures and their expected problems (Ramesh, 2009).

Risk/expected problem	countermeasures
Overheating of the O-ring	Acquire quartz tube having fixed metal flanges
Use of an alternate pump	Acquire a pump that can operate under optimum conditions

Conclusion

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)

References

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