An inductively coupled plasma (ICP) source is generally excited by an RF coil (also referred to as the ICP antenna) mounted outside the reaction chamber. Energy is coupled and fed into the chamber via a high-frequency oscillator through a matching network, which is isolated from the interior of the chamber by a dielectric window. When seed electrons absorb sufficient energy from the alternating electromagnetic field induced by the RF power supply near the coil to exceed the breakdown energy threshold of the process gas, the low-pressure gaseous medium inside the chamber undergoes breakdown and discharge. Thereafter, electrons continuously gain energy in the alternating electromagnetic field, multiply via impact excitation and avalanche ionization, and eventually form and sustain a stable plasma discharge.

ICP discharge is initially dominated by the capacitive coupling mode (E-mode), where energy mainly originates from the electrostatic field formed between coil turns or between the antenna and the grounded chamber wall. In this mode, the plasma sheath is thick with drastic potential fluctuations, which easily causes high-energy ion bombardment and erosion of the dielectric window. As the input RF power rises, the discharge gradually transitions to the inductive coupling mode (H-mode). In H-mode, electron heating is governed primarily by the induced electromagnetic field; the plasma sheath becomes much thinner, and plasma density rises significantly, typically exceeding 10¹⁰ cm⁻³. Although residual capacitive coupling may persist under H-mode and trigger adverse effects such as wall sputtering, this interference can be effectively suppressed by installing a Faraday shield. Precise graded regulation of RF power enables controllable dynamic switching between E-mode and H-mode.

Restricted by electromagnetic damping inside the plasma, the induced current excited by the alternating electromagnetic field concentrates mainly on the plasma surface layer and participates in electron heating only within a limited spatial range — this phenomenon is known as the skin effect. The skin depth is defined as the penetration thickness at which the induced electric field intensity decays to 1/e of its peak value at the chamber boundary. Optimal electron energy absorption and power coupling efficiency are achieved when the skin depth is comparable to the characteristic dimension of the chamber. If plasma density increases further to make the skin depth far smaller than the chamber size, the effective penetration capacity of the electromagnetic field is limited, leading to reduced coupled power.

Figure 1 Schematic diagram of ICP etching system: (a) planar coil ICP; (b) cylindrical coil ICP

ICP sources are classified into two main types based on coil geometry: planar coil ICP and cylindrical coil ICP, as illustrated in Figure 1. For planar coil ICP, the induced electric field distributes mainly along the azimuthal direction. The electric field peaks at a position roughly half the coil radius away from the coil center and drops to nearly zero at the central axis (see Figure 2). In such configurations, the substrate stage is placed far from the antenna, with a separation of 2.5 to 10 times the skin depth. Compared with the main discharge zone, energy dissipation processes such as electron-ion recombination and neutral particle collisions are greatly weakened in this region, which helps improve plasma reaction efficiency on material surfaces.

In contrast, the induced electric field of cylindrical coil ICP reaches its maximum near the chamber wall and attenuates gradually toward the chamber center, readily resulting in uneven spatial plasma distribution. In addition, stable discharge requires a specific geometric matching relationship between antenna coil length and chamber radius, imposing limitations on structural design and scale-up. By comparison, planar coil ICP features flexible diameter scaling; plasma size can be effectively expanded by adjusting coil dimensions, delivering prominent engineering advantages for large-area wafer etching and mass production.

Figure 2 Planar ICP: (a) electromagnetic field schematic; (b) top-down view of electric field distribution

In an ICP system, plasma density is determined collectively by electron heating and collisional ionization within the plasma bulk, regulated primarily by the RF signal fed to the ICP antenna. The RF power supply typically operates at frequencies ranging from 1 MHz to 200 MHz, with 13.56 MHz and its harmonics widely adopted in industrial applications. Ion incident energy and momentum are governed by the potential drop across the sheath, independently tuned by an RF bias power supply connected to the substrate stage, with common bias frequencies including 2 MHz, 13.56 MHz and their harmonics. This separated control mechanism for bulk plasma and sheath regions allows quasi-decoupled regulation of plasma density and ion momentum within a certain operating window — one of the core advantages of ICP over conventional capacitively coupled plasma (CCP).

In summary, the initiation and maintenance of ICP discharge rely on electron heating and collisional ionization driven by RF-induced electromagnetic fields. Discharge evolves from the initial capacitive E-mode to the inductance-dominated H-mode, and is significantly affected by electromagnetic characteristics including the skin effect. Antenna structure and chamber geometry modulate electromagnetic field distribution and particle transport, further determining plasma density magnitude and spatial uniformity. Meanwhile, the capacity of ICP to decouple plasma density and ion momentum under specific operating conditions forms the physical foundation for ICP etching processes.