plasma density

Plasma density refers to the concentration of charged particles (ions and electrons) per unit volume in a plasma, typically expressed in units of ions/cm³ or electrons/cm³. In semiconductor plasma etching and deposition systems, plasma density is a critical parameter that directly influences process characteristics including etch rate, deposition rate, film quality, and pattern transfer fidelity. Plasma densities in semiconductor processing tools vary significantly by source type: capacitively coupled plasma (CCP) reactors generate relatively low densities of 10⁹ to 10¹⁰ cm⁻³, while high-density plasma sources such as inductively coupled plasma (ICP), electron cyclotron resonance (ECR), and helicon wave sources achieve densities of 10¹¹ to 10¹² cm⁻³. The higher plasma density in ICP and ECR systems produces greater concentrations of reactive radicals and ions, enabling faster etch rates at lower pressures with reduced ion bombardment energy, which improves selectivity and reduces damage. In modern etch tools, plasma density and ion energy are independently controlled through separate source power (controlling density) and bias power (controlling energy) RF generators, allowing process optimization across a wide parameter space. Plasma density is measured using Langmuir probes, microwave interferometry, or optical emission spectroscopy (OES) actinometry. Uniformity of plasma density across the wafer is essential for uniform etch rate and CD control — density variations lead to center-to-edge etch rate differences. Factors affecting plasma density include gas composition and pressure, RF power and frequency, magnetic field configuration, and chamber geometry. At very high densities, electron-ion recombination and gas heating can create nonlinear effects. Pulsed plasma operation, where the RF power is modulated between high and low states, provides additional control over plasma density and ion energy distribution, enabling improved selectivity and reduced charging damage in high-aspect-ratio etching.