Semiconductor manufacturing is a high-precision microfabrication system. Core processes such as photolithography, etching, thin-film deposition and ion implantation are all carried out in sealed vacuum chambers. As the only viewing port on the chambers of semiconductor equipment, transparent windows enable engineers to monitor internal conditions in real time during operation, so as to guarantee process yield and equipment safety. However, the operating environment of modern semiconductor equipment chambers has grown increasingly harsh. Transparent windows are constantly bombarded by fluorine-based and oxygen-based plasmas, high-energy ion beams and ultraviolet radiation, as well as exposed to corrosive chemical substances. Meanwhile, the chambers frequently undergo rapid heating and cooling cycles from room temperature to hundreds of degrees Celsius, and endure persistent pressure differentials between the internal vacuum and external atmosphere. Traditional fused silica glass and conventional optical glass can no longer meet the requirements of high-end manufacturing.
Against this backdrop, high-performance transparent ceramics have become the mainstream core material for transparent windows of semiconductor equipment, thanks to their outstanding corrosion resistance, high-temperature stability, radiation resistance, high mechanical strength and excellent light transmittance. This article reviews common transparent ceramic window materials for semiconductor equipment, providing a reference for material selection and practical application.
Transparent Ceramic Materials for Semiconductor Equipment Windows
Unlike ordinary opaque ceramics, transparent ceramics are produced using ultra-pure, ultra-fine ceramic powders as raw materials. Adopting sophisticated fabrication techniques such as atmosphere sintering, hot-press sintering and vacuum sintering, they achieve near-theoretical density and a fully dense structure, which eliminates light scattering caused by pores. In addition, grain control technology is applied to ensure uniform grain size, thin grain boundaries and no impurity segregation, drastically reducing light scattering at grain boundaries. Transparent single crystals can also be fabricated via crystal growth methods, featuring well-ordered lattices and extremely low defect density to minimize light scattering loss.
Not all ceramic powders are suitable for manufacturing transparent ceramics. Currently, cubic crystal system materials including yttria (Y₂O₃), yttrium aluminum garnet (YAG) and aluminum oxynitride (AlON) are widely used. These materials are optically isotropic without birefringence and deliver excellent light transmittance. Single-crystal materials such as sapphire with a hexagonal crystal structure also feature regular lattices and low defect density, achieving high and uniform light transmittance with low optical distortion.
1. Yttria (Y₂O₃) Transparent Ceramics
Yttria transparent ceramics feature an extremely broad optical transmission band, ranging from deep ultraviolet (0.25 μm) to mid-infrared (8 μm). It is one of the few oxide materials that transmit light in both deep ultraviolet and mid-infrared regions. High-quality yttria transparent ceramics generally achieve a visible-light transmittance of 70% to 85%.
Beyond superior optical performance, yttria (Y₂O₃) exhibits exceptional resistance to fluorine-based plasma corrosion. The bond energy of Y–O bonds reaches 780 kJ/mol, rendering the material chemically inert to halogen elements, especially fluorine and chlorine. Even when reacting with fluorine, it forms a stable and dense yttrium fluoride (YF₃) layer that resists peeling. This characteristic greatly reduces particle generation and prevents further erosion of chamber components such as cavities and shower heads by fluorine plasma, thus extending service life. In typical high-density fluorine-based or chlorine-based plasma environments, its etching rate is merely 1/20 to 1/50 of that of quartz glass. With a melting point of 2430 °C, yttria can maintain structural integrity without volatilization or deformation under long-term service above 1750 °C, adapting to the extreme temperature conditions of plasma processes. It is an ideal candidate for transparent windows of high-end etching equipment.
Its main drawbacks lie in relatively low mechanical strength and hardness (Mohs hardness: 6.5–7). It has inferior impact resistance and pressure fatigue resistance compared with sapphire, making it unsuitable for operating conditions involving high pressure and severe impact.
2. Yttrium Aluminum Garnet (YAG)
YAG is synthesized by doping yttria with a certain proportion of alumina. Like yttria, it has a cubic crystal structure, combining the mechanical stability of alumina with the high-temperature resistance and optical advantages of yttria. With well-balanced overall performance, YAG is also more cost-effective than pure yttria. Optically, pure YAG crystals deliver high transmittance across the wavelength range of 0.25 μm to 5 μm, with a visible and near-infrared transmittance of over 80%. Notably, it shows no light absorption in the 2 μm–3 μm band, making it applicable for working conditions involving moderately intense fluorine plasma, high temperatures and vacuum pressure differentials.
3. Aluminum Oxynitride (AlON)
AlON transparent ceramics offer excellent optical transmittance from ultraviolet and visible light to mid-infrared bands, with a transmittance above 80% in the wavelength range of 0.2 μm to 6.0 μm. It boasts a Vickers hardness of 17–18 GPa and a flexural strength of approximately 300 MPa. Featuring high hardness and decent toughness, AlON has excellent resistance to impact and scratching. However, its production requires sintering at 1750 °C to 1900 °C, leading to high energy consumption and manufacturing costs.
4. Transparent Alumina Ceramics
Transparent alumina ceramics achieve a high transmittance of over 90% in the visible and near-infrared bands (200–2200 nm), meeting the optical monitoring requirements of semiconductor equipment. They can also operate stably at temperatures above 1000 °C. In terms of production, transparent alumina ceramics benefit from mature manufacturing processes. They can be fabricated via pressureless sintering, hot-press sintering and other methods, featuring low raw material costs, a wide process window and easy large-scale production.
Nevertheless, the bond energy of Al–O bonds is about 498 kJ/mol. While the material resists most chemical corrosion, it readily reacts with fluorine under high-energy plasma conditions to form aluminum fluoride (AlF₃). This fluoride layer tends to peel off and crystallize on the surface, producing particulate contaminants that may fall onto and contaminate wafers. Meanwhile, the protective layer is continuously consumed, which limits its application in etching equipment.
5. Sapphire
Sapphire is single-crystal alumina, fundamentally different from polycrystalline transparent alumina ceramics. Aluminum and oxygen atoms are regularly bonded via covalent bonds inside the crystal, forming a compact and well-ordered lattice. The single-crystal growth process yields extremely low defect and impurity content, with no grain boundaries or pores that cause light scattering. Light travels through sapphire with minimal obstruction, effectively reducing light scattering and absorption. Therefore, sapphire delivers outstanding optical performance, with a transmission wavelength range of 0.19 μm to 5.5 μm and a straight visible-light transmittance up to 86%, making it an optimal substrate for high-performance optical windows.
However, the single-crystal structure also has obvious disadvantages. Residual internal stress is prone to form during crystal growth, so sapphire is susceptible to brittle fracture and cleavage cracks during use and assembly, which hinders precision lamination and processing. In addition, single-crystal growth involves complex techniques and a low yield rate, resulting in a much higher overall production cost than polycrystalline transparent alumina ceramics.