Recently, graphene has gained significant interest owing to its outstanding conductivity, mechanical strength, thermal stability, etc. Among various graphene synthesis methods, atmospheric pressure chemical vapor deposition APCVD is one of the best syntheses due to very low diffusivity coefficient and a critical step for graphene-based device fabrication. High-temperature APCVD processes for thin film productions are being recognized in many diversity technologies such as solid state electronic devices, in particular, high quality epitaxial semiconductor films for silicon bipolar and metal oxide semiconductor MOS transistors. Graphene-based devices exhibit high potential for applications in flexible electronics, optoelectronics, and energy harvesting. In this chapter, recent advances of APCVD-based graphene synthesis and their related applications will be addressed.
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Epitaxy means "on top" or "assigned to", and represents a process in which a layer is created on top of another layer and inherits its crystal structure.
If the deposited layer is of the same material as the substrate one speaks of homoepitaxy, if it's another material it's so-called heteroepitaxy. The most significant process in the homoepitaxy is the deposition of silicon on silicon, in heteroepitaxy usually a silicon layer is deposited on an insulator such as oxide Silicon On Insulator: SOI.
Depending on the process, the wafers can be delivered from the wafer manufacturer with an epitaxial layer e. As a gas for generating the epitactical layer, pure hydrogen is used in conjunction with silane SiH 4 , dichlorosilane SiH 2 Cl 2 or silicon tetrachloride SiCl 4. The silicon inherits the structure of the substrate and is growing, for energy reasons, layer by layer successively on.
To not grow up a polycrystalline silicon, one must always prevail a shortage of silicon atoms, e. When silicon tetrachloride is used, the reaction proceeds in two steps:.
In order to inherit the substrate's orientation the surface must be absolutely clear. So one can utilize the equilibrium reaction. Both reactions can occur in the other direction, depending on the ratio of the gases. If there is only few hydrogen in the atmosphere, as in the trichlorosilane process for the purification of raw silicon, material is removed from the silicon wafer surface due to the high chlorine concentration.
Only with increasing concentration of hydrogen growth is achieved. With SiCl 4 the deposition rate is approximately 1 to 2 microns per minute. Since the monocrystalline silicon grows only on the bare surface, certain areas can be masked with oxide where the silicon grows as polycrystalline silicon. This polysilicon, however, is etched very easily compared to single-crystalline silicon through the backward-running reaction. Diborane B 2 H 6 or phosphine PH 3 are added to the process gases, to create doped layers, since the doping gases decompose at high temperatures and the dopants are incorporated in the crystal lattice.
The process to create home-epitactical layers is realized under vacuum atmosphere. As mentioned above, due to a low hydrogen concentration there occurs a back etch on the silicon surface. This can be used to clean the surface before the actual process starts.
If the gas concentration is varied post this cleaning the deposition begins. Due to the high process temperatures there's a diffusion of dopants in the substrate or impurities, which have been used in earlier processes, can move to the substrate.
If SiH 2 Cl 2 or SiH 4 are used there's no need for such high temperatures, so these gases are used primarily. To achieve the etch back process to clean the surface, HCl has to be added separately.
The disadvantage of this silanes is that they form germs in the atmosphere right before deposition, and thus the quality of the layer is not as good as with SiCl 4. There is often a need of layers which can't be created right from the substrate. To deposit layers of silicon nitride or silicon oxynitride one has to use gases which contain all necessary components. The gases are decomposed via thermal energy. That's the principle of the chemical vapor phase deposition: CVD.
The wafer surface doesn't react with the gases but serves as bottom layer. Depending on the process parameters - pressure, temperature - the CVD method can be devided in different methods whose layers differ in density and coverage.
If the growth on horizontal surfaces is as high as on vertical surfaces the deposition is conform. A high conformity can only be achieved by high process temperatures. The deposited oxide has a low density and the coverage is moderate due to a relatively low temperature.
The high wafer throughput is a big advantage of this process. As process gases silane SiH 4 highly deluted with nitrogen N 2 and oxygen O 2 are used. Added ozone O 3 can cause a better conformity because it improves the movability of the accumulated particles. The oxide is porous and electrical instable and can be densified by a high temperature process. To avoid edges which can result in difficulties at the deposition of additional layers, phosphorus silicate glass PSG is used for interlayers.
A high amount of phosphorus leads to a high increase of the flow properties, however, phosphoric acid can be formed which corrodes aluminum conductor paths. Because annealing affects earlier processes e.
LPCVD processes enable a high conformity of almost 1. The particles dispread due to collisions and cover vertical surfaces as well as horizontal ones. A tungsten film can only be fabricated on bare silicon. Therefore silane has to be added if there is no silicon substrate. Due to low temperatures the process gases can not be decomposited thermal. With a high frequency voltage, the gas is transformed into a plasma state.
The plasma is energetic and disposes on the surface. Instead of SiH 2 Cl 2 silane is used because it decomposes at lower temperature. The process uses several gases which are led into the process chamber alternating. Each gas reacts in such a way that the current surface is saturated, and therefore the reaction comes to a standstill.
The alternative gas is able to react with this surface in the same way. Between the reactions of these gases the chamber is purged with an inert gas, like nitrogen or argon. A simple ALD process could look like this:. First step is the elimination of hydrogen atoms which are bound to oxygen at the wafer surface. The remaining molecules bond with the unsaturated oxygen. The chamber is purged and subsequent water steam is led into the chamber.
Ever one hydrogen atom of the H 2 O molecules now can react with the former deposited surface atoms to form methane, while the hydroxyl anion is bond to the aluminum atoms.
Hence, there are new hydrogen atoms at the surface which can react in a afterwards step with TMA like in the beginning. The atomic layer deposition provides significant advantages over other deposition techniques, and therefore it's a very important process to manufacture thin films. With ALD even 3-dimensional structures can be deposited very uniform.
Insulating films are possible as well as conductive ones, which can be created on differet substrates semiconductors, polymers, The film thickness can be controlled very precise by the number of cycles. Since the reactive gases are not led into the chamber simultaneously, they can not form germs right before the actual deposition. Thus the quality of the films is very high. Homoepitaxy Depending on the process, the wafers can be delivered from the wafer manufacturer with an epitaxial layer e.
Inhalt Plasma, the fourth aggregation state of a material Chemical vapor deposition Physical deposition methods.
Atmospheric Pressure Chemical Vapor Deposition of Graphene
Epitaxy means "on top" or "assigned to", and represents a process in which a layer is created on top of another layer and inherits its crystal structure. If the deposited layer is of the same material as the substrate one speaks of homoepitaxy, if it's another material it's so-called heteroepitaxy. The most significant process in the homoepitaxy is the deposition of silicon on silicon, in heteroepitaxy usually a silicon layer is deposited on an insulator such as oxide Silicon On Insulator: SOI. Depending on the process, the wafers can be delivered from the wafer manufacturer with an epitaxial layer e. As a gas for generating the epitactical layer, pure hydrogen is used in conjunction with silane SiH 4 , dichlorosilane SiH 2 Cl 2 or silicon tetrachloride SiCl 4. The silicon inherits the structure of the substrate and is growing, for energy reasons, layer by layer successively on. To not grow up a polycrystalline silicon, one must always prevail a shortage of silicon atoms, e.
The series APCVD system is well suited for low cost high-volume processing for all cell architectures requiring single or multi-layer dielectric thin film deposition. In the roller transport system, only the substrate is heated during the process, so electricity and cooling requirements are greatly reduced. Three tiers of graded, power saving insulation further reduce energy bills. The inline system is available in two configurations: with belt transport or with roller transport. Each transport method has its advantages. The world class HMI allows the customer to easily monitor and control all process parameters in a user-friendly touchscreen environment. The maintenance-conscious design allows chemical injectors and exhaust ducting to be cleaned while in place on the system and without significant process interruption.