**2. Chemical vapor deposition**

The chemical vapor deposition technique is based on the chemical thermal decomposition of a precursor (the compound that provides the feedstock for the production of the desired material), so their individual constituents can build up a solid film or nanostructure into a specified substrate. In general, a common CVD equipment is composed of three main modules: 1) a precursor injection module, 2) the reaction site or chamber, and 3) the gas ejection module [5]. Although these three modules can be arranged either vertically or horizontally, the horizontal configuration is the most widely used. The reaction site consists of a region subjected to high temperatures where the thermal decomposition of the precursor can be triggered. A very common setup consists of a tubular furnace with a borosilicate/quartz/alumina tube inside serving as reaction chamber, see **Figure 1**, the nature of the tube would depend on the temperature conditions used in the process. If the process requires a catalyst, it can be placed inside the reaction chamber, such is the case for the CVD synthesis of graphene as will be reviewed in detail below. Depending on the mechanism used for the delivery and the type of precursor, the pressure conditions employed, different CVD configurations can be considered [5]. Each one of these alternatives can undergo the same tasks, but each will use a different approach to fulfill it. In the following sections, some of the most common and popular configurations will be exposed.

#### **2.1 APCVD**

The atmospheric pressure chemical vapor deposition (APCVD) is one of the simplest configurations found for a CVD system since it operates in normal conditions (atmospheric pressure), making this option more affordable and easier to implement at laboratories. However, due to these conditions, problems related to the dispersion of the gases within the chamber have been considered as one of its major disadvantages, because the decomposed particles will not cover the substrate homogeneously as expected [6, 7].

For this approach, different precursors can be used. In the case of liquids or solids dissolved in solution, a bubbler or an evaporation system will be required to stimulate the particles to be expulsed and consequently be transferred into the chamber by the action of a carrier gas, provoking its decomposition in the reaction site [6].

*Chemical Vapor Deposition Synthesis of Graphene on Copper Foils DOI: http://dx.doi.org/10.5772/intechopen.106058*

#### **Figure 1.**

*Schematic representation of a horizontal CVD setup, specifying the three constituent modules (top) and diagram of an LPCVD setup for the synthesis of graphene on copper foil and methane as precursor (bottom), dashed rectangles highlight the corresponding modules.*

#### **2.2 LPCVD**

The low-pressure chemical vapor deposition (LPCVD) is based on the use of vacuum for the outgassing of the system to achieve low pressures throughout the deposition process. In comparison with the last approach, with this technique, the dispersion and the deposition are enhanced, making it possible to obtain better coverage within the substrates and a higher quality. The pressures commonly used are between 10−1 and 10−2 torr and are reached with the use of mechanical and molecular vacuum pumps connected to the gas ejection module [6].

#### **2.3 AACVD**

The aerosol-assisted chemical vapor deposition (AACVD) is characterized for using fine sub-micrometer-sized aerosol droplets of precursor dispersed throughout a gaseous medium, which eventually are transported into the reaction zone to be decomposed and deposited in the target substrate. The atomization of the liquid precursor can be achieved by different routes, as it could be the use of an ultrasonic aerosol generation, a pneumatic aerosol jet, or *via* an electrostatic atomization. The AACVD possesses the advantage that thermally unstable and nonvolatile precursors can be employed for the synthesis of CVD products at lower cost. In addition, this variant is considered a more flexible option since it can be carried out under low pressure, atmospheric pressure, and in open atmosphere [8].

#### **2.4 Roll-to-roll CVD**

Roll-to-roll CVD is one of the most recent approaches developed, and it consists of a continuous process that can carry out the deposition of the material on a substrate and its withdrawal or a sequential process such as a further transference into another

suitable substrate (in some cases it is required). This CVD variant is equipped with conveyor belts that are in charge of the transport of the substrates into the reaction site, where a continuous inlet of gases throughout the chamber leads to a constant deposit built up in the incoming substrate without interruption. Subsequently, the conveyor belts either introduce the material into a second stage, where a transfer procedure can be done automatically, or take the substrate out of the system. This CVD configuration can be developed for different process conditions, such as ambient pressure, low pressure, or a plasma-enhanced CVD [9, 10].

#### **2.5 Other configurations of CVD (PECVD, UHV-CVD, CW-CVD)**

One of the great advantages of using a CVD system is the wide variety of possible configurations. Above, we have described the most popular ones, but there is still a large catalogue of variants, among which are worth mentioning the plasma-enhanced chemical vapor deposition (PECVD), the ultrahigh-vacuum chemical vapor deposition (UHV-CVD), and the cold-wall chemical vapor deposition (CW-CVD), these alternatives will be briefly discussed in the following lines.

The PECVD variant uses plasma to improve the decomposition of the precursor and its deposition, allowing a reduction in the temperature and the possibility of avoiding the use of a catalytic substrate for the deposit [11]. The UHV-CVD is characterized by the use of more extreme low-pressure conditions (~10−7 torr) to avoid the oxidation of the deposit and to enhance the decomposition of the precursor [12]. Finally, the CW-CVD owes its name to the lack of a heating furnace, and it relies on a Joule heating approach for the decomposition of the precursor, achieving a more selective and controllable reaction site, by locally heating the substrate [13, 14].
