Material deposition methods fall into two broad categories: vapour and liquid phase techniques depending on the physical state of the carrying medium which is used to transport the precursor to the substrate.
This section describes the main characteristics of vapour phase deposition (VPD) which is the process whereby gas phase species are condensed, chemically reacted or converted into solid deposits. Vapour deposition techniques are divided into two main categories: physical vapour deposition and chemical vapour deposition. Due to the absence of surface tension in gases, they can completely wet complex topologies. Therefore, vapour precursors provide a powerful medium for the conformal deposition within mesoporous substrates [118]. During the last decades the deposition of high quality thin films, free standing structures and mesoporous materials by vapour deposition methods have been demonstrated. As will be described in the following sections, our research group has developed high-pressure vapour phase techniques for growing amorphous and crystalline silicon and germanium wire/tubes within MOFs [16].
3.2.1
Physical Vapour Deposition
Physical Vapour Deposition (PVD)is typically a direct line-of-sight technique in which the gaseous precursor is produced by either evaporating or ablating the target material.
Chapter 3. Materials Deposition inside Microstructured Optical Fibres 34 The evaporated atoms or molecules solidify onto a substrate and form films. In order to obtain high quality deposits and to allow the particles to travel as freely as possible, the whole process takes place in a vacuum chamber. Many PVD techniques can produce high purity films and have found increased use in the microelectronics and optoelectronics industries, especially for the fabrication of film resistors and filters. The most common PVD methods are: thermal vapour deposition, sputtering, laser ablation deposition and molecular beam epitaxy (MBE) which is particulary useful for top-down micro/nano fabrication, where device quality is at a premium.
3.2.1.1 Thermal Vapour Deposition (TVD)
Thermal evaporation represents one of the oldest of the thin film deposition techniques and, at present, many different kinds of films in integrated circuits are deposited by evap- oration. The material to be deposited is heated to its melting temperature in a vacuum, which allows its vapour to reach the substrate surface and grow thin films [119]. The material is usually evaporated by passing current through a high melting temperature metal (e.g., a tungsten boat or filament). In industrial applications, resistive heating has been replaced by electron beam and RF induction evaporation [118]. TVD is suitable for film growth of materials with simple stoichiometry, making it very common for metal deposition. Indeed, a wide range of metals and alloy thin films such as Au, Al, Ag, Ni, Ni-Cr [120], Ti-Al-V, Ni-Cr-Al [121] have been effectively deposited. This technique is not suitable for compound materials or materials with complex stoichiometries because the different vapour pressures of their constituents would lead to deposits with incor- rect compositions. TVD offers almost no control over the path taken by the evaporated atoms, with consequent slow growth rates, and inefficient use of the source material. Some variations of TVD, such as MBE, offer great control of the deposition process even down to the level of atomic monolayer precision. However, because the process has to take place in a vacuum chamber, the vapour cannot wet the internal walls of mesoporous templates. A representation of a TVD chamber is illustrated in Figure 3.3.
3.2.1.2 Sputtering
A schematic of a sputtering chamber is shown in Figure 3.4. During the deposition pro- cess, the solid coating material (target), at a high negative potential, is converted into vapour phase by ion bombardment (Ar+). The vaporized material is condensed onto the substrate placed on the anode [119]. Although sputtering coating always takes place in the presence of plasma, the pressure in the vacuum chamber is always kept very low and the evaporated atoms follow a more or less collision-less path to the substrate. Sputter- ing is preferred over evaporation in many applications due to the fact that virtually any
Chapter 3. Materials Deposition inside Microstructured Optical Fibres 35
Figure 3.3: Illustration of a typical TVD deposition chamber. Only materials
with a much higher vapor pressure than the heating element can be deposited without contamination of the film. TVD requires good vacuum during evapo-
ration, and pressures lower than 10−6Pa are normally used.
material can be sputtered and has better adhesion to the substrate. This technique is commonly used for the deposition of metals, semiconductors and dielectrics onto inte- grated circuit wafers. An important advantage of the sputtering process is its capacity to convert the material into gas phase without changing its chemical composition, resulting in high purity deposits.
PVD methods have many features which have permitted them to be widely used at laboratory and industrial levels. In PVD, the starting materials are chemically pure and the deposition takes place in a vacuum chamber, thus these methods usually have fewer contamination problems than other deposition techniques. However, since the evapo- rated particles tend to follow a straight path (line-of-sight), films deposited by physical means are commonly directional rather than conformal which prohibits them from form- ing homogeneous films within mesoporous structures. Moreover, since PVD are vacuum deposition processes, there is limited mass transport down the extreme aspect-ratio cap- illaries of MOFs. Thus, PVD techniques cannot be applied to our purposes and chemical methods are must be considered.
3.2.2
Chemical Vapour Deposition
Chemical Vapour Deposition (CVD) is a non-line-of-sight deposition process whereby chemical reactions produce solid material from gaseous reagents on or near the surface of a heated substrate. The constituents in the vapour phase are typically diluted with an inert carrier gas and react at the surface of hot substrates [122]. CVD has developed into one of the most versatile and important methods for material synthesis and today has application to a wide spectrum of compounds. CVD plays a central role in the synthesis
Chapter 3. Materials Deposition inside Microstructured Optical Fibres 36
Figure 3.4: Schematic diagram of a simple sputtering deposition chamber.
of amorphous, polycrystalline and epitaxial single crystal films which are used in almost every semiconductor device. Moreover, CVD techniques are used for the fabrication of conventional step index optical fibres [123].
Figure 3.5: A schematic diagram of a hot wall open flow CVD system. All
substrates can be covered homogeneously because of the low surface reaction rate of the growth process. The total pressure in the chamber is usually less
than 60 kPa.
A simple CVD system apparatus is illustrated in Figure 3.5. The deposition process involves a dynamic flow, in which gaseous precursors enter to the reaction chamber and pass over the heated substrates. If the substrate temperature is high enough, the vapour precursors heterogeneous chemical react to produce a solid coating (deposit) plus by-product gases. The by-products are desorbed and, together with any remaining reactant gas, exit from the hot reaction zone through the exhaust tube. Heterogeneous CVD reaction take place on the substrate surface and have reactants in two or more phases and lead to thin coating. However, the reactions may not always take place on or close to the substrates but also can occur in the gas phase. These second kind of
Chapter 3. Materials Deposition inside Microstructured Optical Fibres 37 CVD reactions, in which the reactants are only in the gas phase are called homogeneous reaction and take place via direct chemical decomposition of the precursors in the gas phase. Homogeneous reactions lead to powders, low density and poorly consolidated films and thus, are not usually desirable [124]. The various transport and reaction processes underlying CVD can be summarized as follows [118]:
• Mass transport of reactants and diluent gases, from the reactor inlet to the depo- sition zone.
• Homogeneous gas phase reactions, leading to film precursors and by-products. These reactions are undesirable.
• Mass transport of reactants to the hot substrate.
• Adsorption of film precursors and reactants on the surface.
• Heterogeneous surface reactions occurring selectively on the heated surface.
• Formation of film nucleation points and film growth.
• Desorption of by-products of the surface reactions.
• Mass transport of by-products in the gas flow away from the reaction chamber. In CVD there are several factors which determine the rate, kind of deposition and the compound deposited, these include gas pressure, precursor concentration, substrate chemistry and temperature, gas precursor flow rate, etc. Because of the many variables involved in CVD and the complicated chemical, thermodynamic and physical phenomena involved, most reaction chambers are designed empirically and the parameters of the deposition are experimentally optimized specifically for each chamber and the desired deposition characteristics. When the CVD parameters are controlled in the correct manner, CVD can lead to the formation of high purity materials with structural control at an atomic or nanometre scale level.
The most popular CVD method uses thermal energy to drive the chemical reactions. However, there exist many diverse low temperature CVD techniques which require an ad- ditional energy source to initiate the reaction. Some examples of these CVD variants in- clude: Laser-assisted (LACVD) and plasma-enhanced CVD (PECVD). Metallo-organic CVD (MOCVD) [125] and aerosol assisted CVD (AACVD) [126] are other important CVD methods.
In LACVD, photons are used either to locally heat (pyrolytic mechanisms) the substrate and initiate the CVD reactions or to decompose (photolytic mechanisms) the precursors in the gas phase. The energy provided by the photons can accelerate the reaction, increase the deposition rate in particular areas of the substrate and lower the deposition
Chapter 3. Materials Deposition inside Microstructured Optical Fibres 38 temperature. The key attraction of this method is direct writing of 3D structures without lithography and extremely low temperature deposition [127]. The LACVD process can be performed at atmospheric or reduced pressure and has been described for a number of materials [128].
In PECVD, an RF-induced plasma transfers energy to the reactant gases. The vapour precursors are ionized and dissociated by electron impacts. The extra energy, provided by the plasma, permits the substrate to remain at a lower temperature, than in thermal CVD, and enhances the deposition rate. PECVD enables dielectric films such as nitrides, oxides, and oxynitrides to be deposited on wafers with small feature sizes which has led to wide use of PECVD in very large scale integration (VLSI).
MOCVD is variation of CVD which relies on the flow of organometallic or hydride gases to deposit materials with complex stoichiometries. MOCVD produces pure films and provides control within one atomic layer. Therefore it has commonly been used to epitaxially grow metallic, III-V and II-VI thin films [129, 125, 130]. This technique plays a key role in the manufacture of many III-V optoelectronic devices, such as, lasers, photodiodes and quantum wells.
Despite the considerable advantages and material flexibility of traditional CVD, the syn- thesis process takes place in reaction chambers at low (∼1 Pa-∼10 Pa) or atmospheric pressures (∼ 100 Pa−10 kPa), making the precursors transport inside the long MOFs not feasible. However, as will be described later, there is no restriction to CVD being extended to higher pressures.