Quote

What is Chemical Vapor Deposition?

Chemical Vapor Deposition (CVD) has become the primary method for producing coated glass, widely used in architectural and industrial applications. In this process, gaseous components react chemically to form solid thin films, which condense on the glass surface as solid-phase reaction products. These reactions occur near or directly on the glass surface (heterogeneous reactions) and must be avoided in the gas phase to prevent the formation of powders. Generally, any controllable reaction capable of producing solid products from one or more vapor phases can be employed for thin-film preparation. For example, common precursors like silicon tetrachloride (SiCl4) and oxygen (O2) react at temperatures of 700–800°C to produce silica (SiO2) coatings.

Offline CVD

CVD processes are categorized into on-line and off-line methods based on the production environment. Off-line CVD, also known as the high-temperature pyrolysis method, involves principles from chemistry, thermodynamics, kinetics, transport mechanisms, film growth phenomena, and reactor engineering. The feasibility of film deposition is determined by the free energy change (ΔG) of the chemical reaction, which is calculated as the difference between the sum of the free energies of the products and the reactants. For example, the ΔG for titanium dioxide (TiO2) formation from titanium tetrachloride (TiCl4) and oxygen at 800°C is approximately -120 kJ/mol, making the reaction highly favorable. The equilibrium constant Kp, which depends on the partial pressures of all components, is directly related to ΔG. In practice, precise control of these parameters is crucial for efficient film deposition.
Several factors, including the diffusion of raw materials to the surface, adsorption of precursors, chemical reactions on the substrate, desorption of by-products, and substrate temperature influence the deposition process. For example, at a substrate temperature of 600°C, the deposition rate of silicon dioxide can be around 0.5 micrometers per minute. According to the Arrhenius equation, the reaction rate increases exponentially with temperature, but at higher temperatures, the process may become limited by the diffusion of reactants or by-products. In such cases, the temperature dependency of the rate often falls within the range of T^-1.5 to T^-2.0. Effective control of these factors ensures optimal deposition rates and film quality.

What is the role of high temperature in offline CVD production?

High temperatures are essential for CVD to produce films with desired properties. Films deposited at temperatures above 800°C are typically polycrystalline, whereas single-crystal films require even higher temperatures, often exceeding 1000°C. At temperatures below 600°C, films are more likely to be amorphous or polycrystalline due to insufficient atomic mobility for crystal growth. For instance, titanium nitride (TiN) films deposited at 900°C exhibit higher density and smoother surfaces compared to those deposited at 500°C. Homogeneous gas-phase reactions, common in conventional CVD, can introduce impurities into the film, degrading its quality. Techniques like plasma-assisted CVD or laser-induced CVD can be used to locally increase reaction energy, ensuring better adhesion and uniformity of films while maintaining process temperatures as low as 400°C for certain materials.

What is the process flow of offline CAD?

The process flow of the offline CVD system is shown in the figure.Gaseous precursors, such as water vapor and titanium tetrachloride (TiCl₄), are pre-mixed with a carrier gas, typically nitrogen, in specific proportions. This mixture is then introduced beneath the coating reaction chamber’s walls. At a controlled temperature near the glass surface, chemical reactions occur, resulting in the formation of solid reaction products that condense on the glass surface as a thin film. By-products of the reaction are expelled through the exhaust system. For instance, in a typical titanium dioxide (TiO₂) coating process, the deposition temperature ranges from 700°C to 800°C, with a gas flow rate of 500–1000 sccm (standard cubic centimeters per minute).

The offline CVD reaction chamber generally adopts a vertical structure. It consists of a tube furnace and a quartz tube, and the resistance wire provides heat. A precision temperature controller keeps the reaction chamber at the desired temperature, and a water cooling system ensures stability. The reaction gas and water vapor are delivered through quartz nozzles, which are designed to withstand high temperatures and chemical reactions. For example, the reaction gas injection rate is usually optimized to 1-3 liters per minute to ensure uniform coating.

On-Line CVD Method

The online CVD method refers to the continuous deposition of chemical compound films on glass substrates during float glass production. The deposition substrate is float glass moving at a speed of approximately 8–15 meters per minute, with a surface temperature of around 600°C. This process occurs just after the glass exits the tin bath but before it enters the annealing lehr. At this stage, the glass is untreated and unpurified, making it ideal for film deposition.

What is the principle of online CVD film formation?

The online CVD method is one of the most advanced techniques for producing coated glass. It involves selecting a temperature zone along the length of the tin bath in the float glass production line that meets process requirements and inserting a coating reactor into this zone. Gas mixtures, composed of reactive gases and carrier gases (usually nitrogen), are pre-mixed in specific proportions and introduced beneath the walls of the coating reactor. At the target temperature, these gases react chemically near the glass surface, forming a solid film on the substrate.
The coating reactor is constructed from high-temperature-resistant materials and cooled with water to ensure long-term stability under the required conditions. By-products of the reaction are expelled through an exhaust system and processed appropriately. The activation energy for the deposition reaction is supplied by the heated glass substrate, and the reaction is conducted under slightly positive pressure.

Schematic diagram of the simplified CVD reactor used for deposition of the Ge thin films.

Reactor Types and Characteristics

The cross-sectional schematic of the deposition system is shown in Figure 4-4. The system consists of the following key components:

1. Reaction gas inlet.

2. Carrier gas inlet.

3. Glass surface (substrate).

4. Exhaust system for residual gases.

5. Identical nozzles for uniform gas delivery.

In the system, reaction gases are transported through controlled-temperature pipes using a carrier gas (typically nitrogen) to a deposition apparatus consisting of multiple narrow nozzles. The gas expands in the first chamber of the nozzle and then accelerates through a narrow slit 0.5 mm wide. The reaction gas exits the nozzle in a laminar flow, mixing with the carrier gas only by diffusion.

The optimal distance between the slit nozzle and the glass surface depends on the total gas flow rate. For instance, when the flow rate is 1 m³/h per meter of nozzle length, the ideal distance is 3 mm.

Figure 4-4 Schematic diagram of the cross section of the reactor deposition device
1—reaction gas inlet; 2—carrier gas inlet; 3—glass surface;
4—residual gas exhaust; 5—same nozzle

Reactor Types and Characteristics

The basic functions of the deposition reactor system include:

  • Controlling the flow of reactive and dilution gases through the reactor.
  • Heating the substrate (glass surface) to maintain a defined temperature.

Safely exhausting residual gases. For atmospheric-pressure CVD on glass, low-temperature reactors are generally used. Based on gas flow characteristics and design, reactors are categorized into four main types, as illustrated in Figure 4-5:

  1. Horizontal Tube Flow Reactor: Gas flows horizontally along a tubular reactor, commonly used for thin-film production over large areas.
  2. Vertical Rotating Bath Reactor: Gas is introduced into a vertically rotating chamber, suitable for intermittent processing.
  3. Expanded Area Diffusion Reactor: Pre-mixed gases flow through a slotted diffusion plate for continuous deposition over an expanded surface area.
  4. Layered Flow Nozzle Reactor: Laminar flow nozzles direct gas streams toward the substrate for precise, continuous deposition.

Characteristics and Advantages of On-Line CVD for Coated Glass Production

The on-line CVD method for coated glass production offers several advantages. It requires simple equipment, such as a coating reactor, gas preparation devices, and exhaust systems, leading to low investment costs, minimal power requirements, and low operational expenses. For instance, the energy consumption is typically under 5 kWh per square meter of glass. The production capacity is scalable to match float glass lines, which can process up to 800 tons per day, ensuring high economic efficiency. Additionally, this method allows for dual-use of the production line, enabling flexible switching between standard float glass and coated glass products based on market demand. Glass produced using on-line CVD is compatible with deep processing methods, such as tempering and thermal bending, without compromising the coating’s durability, adhesion, or color. The resulting coated glass exhibits excellent physical and chemical properties, including resistance to abrasion, acids, alkalis, and salts, making it ideal for demanding applications.

Factors Affecting Quality

The quality of coated glass produced via online CVD is influenced by several factors. Accurate gas mixing ratios are critical; for example, a ±5% deviation in the silane (SiH₄) to hydrogen (H₂) ratio can significantly reduce film adhesion. The gas concentration affects the deposition rate and film thickness, with optimal concentrations enabling uniform coatings. Substrate temperature plays a key role, with the best results achieved in the 580–620°C range; lower or higher temperatures can lead to defects like thin films, pinholes, or bubbles. Consistent glass pulling speeds, typically around 10 meters per minute, are essential for maintaining uniform film thickness. Proper exhaust system design is crucial to remove by-products and unreacted gases without disturbing lthe aminar flow at the glass surface.

Contact Us

Your feedback fuels our growth, and your questions drive our solutions.

We value your feedback, inquiries, and suggestions. Please feel free to get in touch with us

General inquiries

Please contact us via sales@bo-glass.com, and we will reply to you as soon as possible.

Interested to work with us

Drop your resume at info@bo-glass.com
and we will get back to you shortly.

    We uses the contact information you provide to us to contact you about our relevent content, products, and services.

    1