01

Thermal Chemical Vapor Deposition (TCVD)

The principle of thermal chemical vapor deposition (TCVD) is to utilize volatile metal halides and organometallic compounds, which undergo gas-phase chemical reactions at high temperatures—including thermal decomposition, hydrogen reduction, oxidation, and displacement reactions—to deposit desired thin films of nitrides, oxides, carbides, silicides, high-melting-point metals, pure metals, and semiconductors on a substrate. During the process, the constituent precursors are supplied in gaseous form, and the reaction exhaust gases are removed by a pumping system. In addition to heating the substrate to the appropriate temperature via thermal energy (such as radiation, conduction, or induction heating), the gas molecules are also excited and dissociated, thereby promoting the reaction. The dissociation products or reaction intermediates then deposit on the substrate surface to form a thin film.

Thermochemical vapor deposition can be further classified into three categories based on the nature of the chemical reactions involved: chemical transport, pyrolysis, and synthesis reactions. Among these, chemical transport is capable of producing thin films but is generally employed for the growth of bulk crystalline substrates; pyrolysis is typically used for thin-film deposition; and synthesis reactions find applications in both scenarios. Thermochemical vapor deposition is utilized for semiconductor substrates and other materials. Widely adopted CVD techniques, such as metalorganic CVD and hydride CVD, fall within this category.

02

Low-Pressure Chemical Vapor Deposition (LPCVD)

Low-pressure chemical vapor deposition (LPCVD) was developed on the basis of atmospheric-pressure chemical vapor deposition to enhance film quality, improve the uniformity of film thickness and resistivity distribution, and increase production efficiency. The main characteristics of LPCVD include:

(1) The pressure range for low-pressure chemical vapor deposition typically spans from 0.0001 × 10^4 to 4 × 10^4 Pa. At low pressures, the mean free path of gas molecules increases, thereby accelerating the transport of gaseous species and enhancing the diffusion coefficient of reactants at the substrate surface, which in turn improves film uniformity. For epitaxial growth governed by surface-diffusion kinetics, this leads to increased uniformity of the epitaxial layer—a requirement in large-area, large-scale epitaxial processes, such as dielectric-layer epitaxy in mass-produced silicon-device fabrication. However, for epitaxial growth controlled by mass transport, the aforementioned effects are not significant.

(2) Low-pressure epitaxial growth places stringent demands on equipment, requiring a precise pressure-control system and the use of a diffusion-furnace-type reactor that allows for easy temperature control. Under low-pressure conditions, uniform heating of the substrates is more readily achieved, and substrates can be loaded in large batches, thereby significantly enhancing both reliability and production efficiency. In some cases, low-pressure epitaxy is an indispensable technique; for instance, when a chemical reaction is highly sensitive to pressure, reactions that are difficult to carry out at atmospheric pressure become feasible under low-pressure conditions. However, low-pressure epitaxy can sometimes affect the segregation coefficient.

(3) Due to the vertical loading of Si wafers, even as the diameter of the silicon wafers increases, the processing capacity remains unaffected. To further accommodate larger wafer sizes and suppress particle generation, a vertical-type reactor can be employed.

03

Plasma Chemical Vapor Deposition (PCVD)

According to the method of supplying power to the reaction chamber, the PCVD method can be classified into the following categories:

(1) DC method. The technique that uses the activation of chemical reactions by a DC plasma for vapor-phase deposition is known as DC plasma-enhanced chemical vapor deposition (DCPCVD). In this process, film formation occurs on the cathode side, and the resulting film is strongly influenced by the intense magnetic field generated by the space charge near the anode. When the reaction gas is diluted, argon can become incorporated into the film; to prevent this, a shield with a potential equal to that of the cathode-side substrate is positioned in front of the cathode, thereby enabling the deposition of high-quality films.

(2) Radio-frequency method. The technique that uses radio-frequency plasma to activate chemical reactions for vapor-phase deposition is known as radio-frequency plasma-enhanced chemical vapor deposition (RFPCVD). The coupling methods for supplying radio-frequency power are broadly classified into inductive coupling and capacitive coupling. In the discharge process, the electrodes do not suffer corrosion and there is no contamination by impurities; however, the positions of the substrate and the external electrodes must be adjusted. Another approach involves embedding the electrodes within the chamber, with the parallel-plate configuration (capacitive coupling) in particular demonstrating excellent performance in terms of electrical stability and power efficiency, leading to its widespread application. The reaction chamber pressure is maintained at around 0.13 Pa, and a bias voltage is applied between the substrate and the plasma to promote deposition on the substrate surface. The radio-frequency method can be used to deposit insulating films.

(3) Microwave method. The technique of using microwave plasma to activate chemical reactions for vapor-phase deposition is known as microwave-plasma chemical vapor deposition (MWPCVD). Thanks to advances in microwave-plasma technology, it is now straightforward to generate microwave plasmas at a wide range of gas pressures. Several MWPCVD systems are currently available. For example, one approach employs a low-pressure chemical-vapor-deposition reaction chamber in which a resonant cavity is mounted crosswise above the chamber and matched with a corresponding microwave generator. Microwaves at 2.45 GHz are coupled into the chamber via a rectangular waveguide, thereby inducing plasma formation in the gas enclosed by the resonant cavity within the reaction chamber. This results in very high degrees of ionization and dissociation, after which the plasma is directed onto the substrate by an axially symmetric magnetic field. The microwave output power typically ranges from several hundred watts to over 1 kW, with the exact value determined by factors such as the substrate temperature and the growth process, including constraints imposed by mass-transport–limited rate-determining steps. This technique offers the following advantages: (1) further reduction of the substrate temperature, thereby minimizing dislocation defects and interdiffusion of constituents or impurities that can arise during high-temperature growth; (2) avoidance of electrode contamination; (3) reduced plasma-induced damage to the thin film; (4) greater suitability for preparing thin films of low-melting-point materials and materials that are thermally unstable at elevated temperatures; (5) owing to its high frequency, the control requirements for the system’s gas pressure can be significantly relaxed; and (6) due to its high frequency, crystalline diamond is more readily obtained in diamond synthesis. In addition to the three methods described above—DC, RF, and microwave—there is also a technique that applies both electric and magnetic fields simultaneously. This approach leverages the magnetic field to extend electron lifetime, thereby effectively sustaining the discharge; in some cases, the discharge must be carried out under exceptionally low-pressure conditions.

PCVD was initially used to deposit SiO2 on semiconductor substrates using organosilicon compounds; it subsequently found widespread application in the semiconductor industry for the deposition of materials such as Si3N4, silicon, silicon carbide, and phosphosilicate glass. Today, PCVD is no longer confined to semiconductor substrates—it is also employed on metallic, ceramic, and glass substrates to form protective, strengthening, modifying, and functional films. Two other important applications of PCVD are the preparation of polymer films and thin films of diamond and cubic boron nitride, both of which demonstrate promising prospects for future development.

Compared with TCVD technology, PCVD technology has the following characteristics.

(1) Film formation can be achieved at lower temperatures. For example, the deposition of TC, Ti(CN), TiN, and Si3N4 can be carried out at reaction temperatures of 700 K, 550 K, 520 K, and 530 K, respectively, whereas conventional chemical vapor deposition requires temperatures of 1200 K, 1000 K, 900 K, and above 1200 K, respectively. The reason PCVD can operate at lower temperatures is that, in plasma-enhanced chemical vapor deposition, it is not the gas temperature that excites and dissociates the gas molecules, but rather the energy of the electrons in the plasma. In most PCVD processes, a non-equilibrium plasma is employed, in which the electron temperature is very high while the gas temperature remains low—often close to room temperature. Within the glow-discharge regime, the electron temperature of the resulting plasma typically ranges from 1 to 10 eV, which is sufficient to break the chemical bonds between gas atoms, thereby inducing excitation and dissociation of the gas species and generating highly chemically active ions and various reactive chemical groups (atomic clusters). Reducing the reaction temperature in chemical vapor deposition is of great technical significance: many substrate materials, such as aluminum or organic polymers, would melt if exposed to excessively high temperatures, while the latter may decompose, degrade, or undergo outgassing. Furthermore, certain metals and alloys may undergo phase transformations at elevated temperatures; the volumetric changes associated with these structural alterations can generate stresses that lead to cracking or delamination of the deposited film.

Dopant elements used in semiconductor processing, such as boron and phosphorus, exhibit significant diffusion at temperatures exceeding 800°C, which degrades device performance. Plasma-assisted deposition enables the facile formation of various thin films on these doped substrates.

(2) It can significantly reduce the internal stress caused by the mismatch in thermal expansion coefficients between the thin film and the substrate.

(3) Even for substances that are extremely slow to form films via thermal processes, PCVD can still be used to deposit films at a reasonable deposition rate. This is because, in most PCVD systems—particularly those employing glow discharge—the operating pressure is relatively low, which enhances mass transport of reactant and product gases across the boundary layer between the convective flow region and the substrate surface, thereby improving film thickness uniformity. Low deposition temperatures favor the formation of amorphous and microcrystalline films, which often exhibit unique and superior properties. Moreover, for materials with different thermal decomposition temperatures, it is possible to synthesize them in varying compositional ratios. However, PCVD also has certain limitations. First, in the plasma, the energy distribution of electrons is broad; in addition to electron–electron collisions, ion–ion collisions and radiation emitted during discharge can also generate new species. Consequently, PCVD reactions are not always highly selective, and multiple chemical reactions may occur simultaneously, making it difficult to control the reaction products and often complicating the elucidation of the reaction mechanisms. As a result, achieving high-purity products is challenging in PCVD: due to the relatively low deposition temperature, desorption of by-product gases and other gases generated during the reaction is incomplete, leading to their residual incorporation into the deposited film (especially hydrogen). Furthermore, when depositing compounds such as carbides, nitrides, oxides, and silicides, it is difficult to ensure precise stoichiometric ratios. In general, this is detrimental, as it can alter the material’s physical and chemical properties and reduce its corrosion resistance and radiation tolerance. Second, PCVD tends to induce compressive stress within the thin film. For ultrathin films used in semiconductor processing, such stress generally does not pose major problems; for metallurgical coatings, compressive stress can even be advantageous. However, when the coating is relatively thick, this stress may lead to cracking and delamination. Another drawback of PCVD is its potential to cause ion-beam-induced damage to certain fragile substrates, such as III–V and II–VI compound semiconductors commonly used in semiconductor fabrication—particularly when the ion energy exceeds 20 eV. In addition, the plasma can interact strongly with the coating surface during deposition, meaning that both the deposition rate and the resulting film properties depend on the uniformity of the plasma. Finally, PCVD equipment is typically complex and expensive.

Overall, the advantages of PCVD are predominant, and it is now being increasingly widely applied.

04

Metalorganic Chemical Vapor Deposition (MOCVD)

Metal–organic compounds are a class of substances containing carbon–metal bonds. To be suitable for MOCVD, they should be easy to synthesize and purify, remain liquid at room temperature with an appropriate vapor pressure, exhibit a relatively low thermal decomposition temperature that minimizes contamination of the deposited film, and possess low toxicity. Taking the growth of group V–III compound semiconductors as an example, high-purity hydrogen is used as the carrier gas; it carries the vapor of organometallic compounds of group III from a bubbling flask, while group V hydrides diluted with high-purity hydrogen are separately introduced into the reaction chamber. The substrate is placed on a high-frequency-heated graphite susceptor, and the heated substrate exerts a catalytic effect on the thermal decomposition of the metal–organic precursors, leading to the formation of an epitaxial layer under conditions far from thermal equilibrium. Over a broad temperature range, the growth rate is independent of temperature and depends solely on the mass flux of precursor species reaching the surface.

The equipment used in MOCVD technology includes precise temperature control systems, precise pressure control systems, precise gas flow control systems, high-purity carrier gas processing systems, and exhaust gas treatment systems. To enhance the clarity of heteroepitaxial interfaces, a high-speed, dead-volume-free multi-channel gas switching valve is typically installed upstream of the reaction chamber. Furthermore, to ensure smooth gas switching, separate reaction gas lines and auxiliary gas lines are generally provided, with the gas pressures in both lines maintained at equal levels.

Depending on the growth pressure, MOCVD is further classified into atmospheric-pressure MOCVD and low-pressure MOCVD. By integrating MOCVD with molecular-beam epitaxy (MBE), technologies such as metal-organic molecular-beam epitaxy (MOMBE) and chemical-beam epitaxy (CBE) have been developed.

Compared with conventional chemical vapor deposition, the advantages of MOCVD are as follows: (1) lower deposition temperature; (2) the ability to deposit single-crystal, polycrystalline, and amorphous films, as well as ultrathin and atomic-layer films; (3) large-scale, low-cost preparation of complex-composition thin films and compound semiconductor substrate materials; (4) the capability to deposit on various substrate surfaces; and (5) the flexibility to introduce one or more MO precursors to add a specific component or compound to the deposited material, and to use two or more MO precursors to deposit binary or multicomponent, bilayer or multilayer surface coatings, thereby offering broad process versatility. The disadvantages of MOCVD include relatively slow deposition rates, making it suitable only for depositing micrometer-scale surface layers; the high toxicity of the precursor materials; and the need for highly sealed and reliable equipment, along with careful management and operation.

05

Laser-Excited Chemical Vapor Deposition (LCVD)

Compared with conventional chemical vapor deposition, low-pressure chemical vapor deposition (LCVD) can significantly reduce the substrate temperature, thereby preventing disruption of impurity distribution within the substrate and enabling thin-film synthesis on substrates that cannot withstand high temperatures. For example, when preparing SO2, Si3N4, and AlN films by thermal CVD, the substrate must be heated to 800–1200°C, whereas LCVD requires only 380–450°C. Compared with plasma-enhanced chemical vapor deposition (PECVD), LCVD avoids damage to the film caused by high-energy particle irradiation. Since the reactant molecules absorb only photons of specific wavelengths, the selection of photon energy determines which chemical bonds are broken, thus allowing for better control over the film’s purity and structure.