Definition and Basic Characteristics of Sapphire Substrates
A sapphire substrate is defined as a thin sheet processed from the single crystal sapphire (Al₂O₃). The excellent physical and chemical properties of the material have led to its widespread use in optoelectronics, microelectronics and other high-tech fields. The sapphire substrate exhibits a high degree of transparency, high hardness, excellent thermal conductivity and chemical stability, rendering it a pivotal material in many cutting-edge technologies.
Sapphire is essentially a crystalline form of alumina, exhibiting a hexagonal crystal structure and a hardness ranking second only to that of diamond. Its high temperature resistance, mechanical strength, and optical transparency render the sapphire substrate an effective choice for use in harsh working environments, particularly in optoelectronics and microelectronic devices, where it provides a stable and excellent performance.
Preparation of sapphire substrate
- Raw materials
The primary component of sapphire crystal is alumina (Al₂O₃). This material is distinguished by its exceptional physical and chemical properties.
Chemical composition:
Pure sapphire is composed of alumina, but depending on different synthesis methods and application needs, a small amount of impurities may be added to adjust its characteristics.
Physical properties:
Sapphire crystals are characterized by a high hardness (Mohs 9), high light transmittance (performing well from the ultraviolet to the near-infrared spectrum), as well as good thermal conductivity and a low thermal expansion coefficient.
Sapphire substrates are typically manufactured to specific specifications, based on different application and requirements. They are available in a range of thicknesses, typically from a few hundred microns to a few millimeters, and in diameters from approximately 2 to 8 inches, or even larger.
- Synthetic techniques
The following section presents the principal methods employed in the synthesis of sapphire crystals.
- Flame Fusion/ VerneuilMethod
This method is the earliest artificially-synthesized approach to producing sapphire. By melting alumina powder in a high temperature flame, the powder is deposited layer by layer to form a single crystal. This method is characterized by high production speed and low cost. Conversely, the quality of the produced crystal is relatively poor, making this method suitable only for the production of small-size sapphire crystals.
- Kyropoulos Method
The Kyropoulos method is a technique of producing large-sized, high-quality sapphire crystals by slowly cooling molten alumina. This method allows the creation of crystals with low internal stress and few defects, making it an invaluable technique in industrial production.
- BridgmanMethod
The Bridgman method employs gradient heating technology to facilitate the gradual movement of the melt from the high-temperature region to the low-temperature region, so that the crystals can be gradually formed during the cooling process. The method allows the production of high-purity, high-quality single crystals, which are suitable for specific high-precision applications.
- CzochralskiMethod
This method comprises the immersion of seed crystals in molten alumina, followed by a gradual lifting and rotation of the seed crystals to facilitate the gradual crystallization of the melt on them. This method is capable of producing large, high-quality single crystals, which makes it one of the most commonly employed techniques for the production of sapphire substrates.
- Hydrothermal Synthesis Method
Hydrothermal synthesis is conducted under high temperatures and pressures to facilitate the growth of sapphire crystals via chemical reactions in aqueous solutions. This approach is well-suited for cultivating sapphire crystals with special forms and high purity, primarily intended for scientific research and specific industrial applications.
- Processing technology
- Cutting technology:
Sapphire crystals are typically cut into thin sheets by diamond blades or wire saws. The cutting speed and force need to be precisely controlled during the cutting process to avoid cracks and defects in the crystal.
- Polishing and surface treatment
Following the cutting process, sapphire substrates are subjected to a series of polishing stages in order to attain the optimal surface finish and flatness. Polishing powders, such as diamond and alumina, are employed in the polishing process, and mechanical and chemical methods are combined to ensure a smooth and flawless surface on sapphire substrates.
- Size and thickness control
The dimensions and thickness of sapphire substrates play a significant role in determining their potential applications. By employing advanced processing apparatus and precise measuring devices, the dimensions and thickness of the substrates can be accurately regulated to guarantee that it complies with stringent industry standards and application requirements.
