Today, in modern semiconductor etching, thin-film deposition, and other equipment, advanced ceramic materials are gradually replacing traditional metals and polymers due to their excellent high-temperature resistance, corrosion resistance, high insulation, and high hardness. They have become indispensable materials for components such as electrostatic chucks, chamber liners, heaters, robotic arms, and precision guide rails for lithography machines. In plasma etching processes, the focus ring – a critical consumable placed immediately adjacent to the wafer edge – directly affects etching uniformity and chip yield through its material selection. As process windows continue to narrow, two types of ultra-hard ceramics, silicon carbide (SiC) and boron carbide (B₄C), are gradually replacing traditional quartz and silicon materials, becoming key development directions for high-end focus rings.
What is a focus ring?
A focus ring, also known as a confinement ring or edge ring, is an annular precision component installed around the wafer pedestal in a plasma etching system. During plasma etching, the focus ring sits immediately next to the wafer edge and is directly exposed to the high-energy plasma environment. Its core functions are:
(1) Plasma focusing: Semiconductor etching relies on high-energy plasma to precisely etch the wafer. However, at the wafer edge, plasma density tends to decrease due to the electric field edge effect. Through precise structural design and dielectric properties, the focus ring confines and focuses the high-energy plasma over the wafer area, directing it to bombard the wafer surface at a nearly vertical angle. This ensures more uniform plasma distribution across the wafer, reduces etching differences between the edge and center, and improves process uniformity.
(2) Protection of chamber and precision components: During etching, high-energy plasma and highly corrosive etching gases (CF₄, Cl₂, NF₃, etc.) continuously bombard and corrode internal chamber components. The focus ring acts as a first barrier, shielding precision core components underneath such as the electrostatic chuck and electrodes from direct exposure to plasma and corrosive gases, reducing physical bombardment and chemical corrosion damage, and extending the service life of core components.
Material requirements for focus rings and application advantages of advanced ceramics
A focus ring may be continuously exposed to RF plasma for several hours to tens of hours in a single etching process, facing high-density plasma bombardment, fluorine- or chlorine-based corrosive gases, and frequent high-low temperature thermal cycling, while also being in direct contact with the wafer. This requires materials to simultaneously meet stringent demands: extreme plasma erosion resistance, excellent thermal stability and thermal shock resistance, low impurity contamination risk, outstanding mechanical properties, and matched electrical characteristics. In the past, focus rings were mainly made of quartz and silicon. However, as etching processes move toward higher power, the limitations of traditional materials have become increasingly apparent:
Quartz rings: Low cost and good stability under high-frequency electric fields, with excellent electrical insulation. However, they have low hardness (Mohs hardness 7), weak resistance to ion sputtering, a maximum service temperature below 1100°C, susceptibility to deformation at high temperatures, high erosion rates in fluorine-containing plasmas, and high risk of impurity precipitation. They are suitable only for low-to-mid-end RIE etching equipment for nodes above 28nm and cannot meet the low contamination and long life requirements of advanced processes.
Silicon rings: Well-matched thermal expansion coefficient and electrical properties with silicon wafers, and high temperature resistance up to 1600°C, enabling uniform plasma distribution. However, they also have poor resistance to fluorine-containing plasma erosion, easily forming volatile SiF₄, leading to high consumption rates and frequent replacement, which causes process fluctuations and downtime losses. They are suitable only for traditional low-to-mid-end processes.
Against this backdrop, advanced ceramics such as alumina (Al₂O₃), silicon carbide (SiC), and boron carbide (B₄C) have entered the view of semiconductor equipment manufacturers and are gradually becoming the mainstream choice for high-end focus rings.
(1) Alumina (Al₂O₃): Alumina is one of the early ceramics applied in semiconductor equipment, typically with purity above 99.5%, and high-grade products can reach 99.9%. Its preparation process is mature, using pressureless sintering or hot pressing sintering, with significantly lower cost than SiC and B₄C. As a focus ring, it offers high hardness and wear resistance, reducing particle contamination from wear. In fluorine- or chlorine-based plasmas, it forms a stable passivation layer of AlF₃ or AlCl₃, providing good resistance to plasma sputtering. It is suitable for medium-power-density etching processes with a relatively long service life. Additionally, its dielectric properties are stable with good insulation, effectively isolating the electric field and avoiding interference with the electrostatic chuck. However, under high temperature and high fluorine flow, the AlF₃ passivation layer may peel off and become a contamination source. Moreover, its coefficient of thermal expansion (CTE) is about 7.0×10⁻⁶/K, which differs significantly from that of silicon (about 2.6×10⁻⁶/K), potentially causing dimensional changes at high temperatures and affecting alignment accuracy with the wafer, limiting its application in high-precision, small-edge-gap scenarios.
(2) Silicon carbide (SiC): SiC focus rings have become the mainstream upgrade for high-end etchers in recent years. Their Mohs hardness is as high as 9.5, flexural strength remains at 500-600 MPa even at 1400°C, and their CTE (4×10⁻⁶/°C) is close to that of silicon wafers, ensuring stable gaps at high temperatures. They have excellent thermal shock resistance, withstand rapid thermal cycling, and help optimize edge uniformity. More importantly, they exhibit outstanding erosion resistance to Ar, F, Cl, and other plasmas – especially in fluorine plasmas, where the erosion rate is nearly zero. Compared to alumina, they offer longer service life and significantly improved overall equipment efficiency (OEE). SiC focus rings can be produced by pressureless sintering, hot pressing, or chemical vapor deposition (CVD). CVD-produced high-purity SiC can reach purity above 99.9995%, making it suitable for mainstream advanced processes from 5nm to 28nm.
(3) Boron carbide (B₄C): B₄C is an important candidate material in many engineering applications. As early as 2022, Samsung Electronics was already conducting R&D on B₄C focus rings. Earlier this year, Hubei Longzhong Laboratory successfully developed China's first B₄C ceramic focus ring. Compared with mainstream SiC focus rings, it offers 30% higher erosion resistance, a service life exceeding 30 days, reduces etching process costs by about 20%, improves chip manufacturing efficiency and throughput, while maintaining excellent thermal stability and mechanical properties – achieving world-leading technology.