Ion Flux Regulation Accuracy of High Voltage Radio Frequency Bias Power Supply for Atomic Layer Etching Equipment
Atomic layer etching enables precise material removal with atomic scale control by separating the etching process into self limiting steps that remove one atomic layer at a time. This technique is essential for advanced semiconductor manufacturing where feature dimensions approach the nanometer scale and conventional continuous etching cannot achieve the required precision. The ion flux to the substrate surface, controlled by the high voltage radio frequency bias power supply, is a critical parameter that determines the etching rate and the process control.
The atomic layer etching process typically consists of two steps that are repeated until the desired etch depth is achieved. In the first step, a modification step, the surface is exposed to a precursor that modifies the top atomic layer, creating a modified layer with different properties than the underlying material. In the second step, an removal step, ions bombard the surface and remove the modified layer while stopping on the unmodified material beneath. The self limiting nature of the process ensures that each cycle removes approximately one atomic layer regardless of variations in exposure time or ion flux.
The ion flux during the removal step must be controlled within tight tolerances to achieve the desired etching characteristics. Insufficient ion flux may not completely remove the modified layer, leaving residues that affect subsequent cycles. Excessive ion flux can cause physical sputtering of the unmodified material, breaking the self limiting behavior and causing continuous etching with loss of atomic layer control. The ion flux regulation accuracy of the bias power supply directly affects the process window and the achievable etching precision.
Radio frequency bias power supplies generate the ion bombardment by applying an alternating voltage to the substrate electrode. The alternating voltage creates a plasma sheath with a time averaged potential that accelerates ions toward the substrate. The ion energy depends on the amplitude of the radio frequency voltage and the frequency relative to the ion transit time. The ion flux depends on the plasma density and the ion saturation current to the sheath. The bias power supply must control both the voltage amplitude and the power delivered to the plasma to regulate the ion flux.
The relationship between bias power supply parameters and ion flux involves complex plasma physics that depends on the plasma conditions, the electrode geometry, and the gas properties. The ion flux is proportional to the plasma density and the Bohm velocity, which depends on the electron temperature. The plasma density depends on the power absorbed from the bias and from the main plasma source. The coupling between bias power and plasma density varies with pressure, gas composition, and chamber geometry. These relationships must be characterized for specific process conditions to enable accurate flux control.
Flux measurement techniques for process control include ion current measurement at the substrate, optical emission spectroscopy of the plasma, and Langmuir probe measurements. Ion current measurement using a biased electrode or the substrate current provides direct measurement of the ion flux. Optical emission spectroscopy measures the emission from excited species in the plasma, which relates to the plasma density and electron temperature. Langmuir probes provide detailed plasma characterization but may not be suitable for production environments. The measurement accuracy affects the achievable flux regulation.
Control algorithms for ion flux regulation must account for the dynamics of the plasma response to bias power changes. The plasma density responds to power changes with time constants related to the ionization and recombination rates. The control loop bandwidth must be appropriate for the process dynamics. Proportional integral derivative controllers can achieve good regulation for steady conditions. More sophisticated control approaches may be needed for processes with varying conditions or for rapid transitions between different flux setpoints.
Process drift over time affects the relationship between bias power supply settings and the actual ion flux. Chamber wall conditions change with accumulated process deposits, affecting the plasma characteristics. Electrode surface conditions affect the secondary electron emission and the plasma properties. Gas composition may drift due to flow controller variations or gas supply changes. The flux control system must compensate for these drifts through periodic recalibration or adaptive control algorithms that adjust to changing conditions.
The radio frequency matching network that couples the bias power supply to the plasma affects the power delivery efficiency and the flux control. The matching network transforms the plasma impedance to match the power supply output impedance, maximizing power transfer. Changes in plasma impedance with process conditions require matching network adjustment to maintain efficient coupling. The matching network response time affects the ability to maintain flux control during transients. Integrated matching with automatic tuning enables rapid adaptation to changing conditions.
