Etching uniformity is one of the core indicators of plasma etching technology, which directly affects the yield and performance of semiconductor devices. It usually requires multidimensional collaborative control from plasma distribution, process parameters, hardware design, sample state, and chamber maintenance. The following analysis will focus on specific control strategies:
The density, ion energy, and distribution of active groups in plasma are the basis for determining the uniformity of etching, which requires precise control through radio frequency systems, magnetic field assistance, and chamber symmetry design.
1. Fine tuned control of radio frequency (RF) systems
RF power is the core driving force for plasma generation, and its stability and distribution directly affect plasma uniformity
Power stability control: High precision RF power supply (such as ± 0.1% power fluctuation accuracy) is used, combined with an AutoMatching Network, to compensate for impedance changes in the chamber in real time (caused by sample state and gas composition changes), avoiding local plasma "extinguishing" or "excessive strength". For example, in reactive ion etching (RIE), impedance mismatch can lead to a decrease in plasma density at the edge of the wafer, and the etching rate deviation can reach more than 10%. The response speed of the matching network needs to be controlled in milliseconds.
Multi frequency RF collaborative control: using dual frequency (such as 13.56MHz+2MHz) or three frequency RF systems, high frequency (such as 13.56MHz) mainly controls plasma density (affecting etching rate), and low frequency (such as 2MHz) controls ion bombardment energy (affecting etching directionality). By adjusting the power ratio of different frequencies, the "density uniformity" and "energy uniformity" can be optimized separately - for example, for the plasma density difference between the center and edge of the wafer, the density deviation can be reduced to within 3% by increasing the high-frequency power ratio in the edge region (such as using partitioned RF electrodes).
2. Magnetic field assisted plasma confinement
For high-density plasma etching (such as ECR etching, ICP etching), the magnetic field can constrain electron motion through Lorentz force, improving plasma density uniformity:
Optimization of magnetic field strength and distribution: A "uniform magnetic field system" composed of multiple coils is used to adjust the coil current through simulation (such as COMSOL magnetic field simulation), so that the deviation of magnetic field strength in the chamber is ≤ 5%. For example, in ICP etching, if the magnetic field center is too strong and the edge is weak, it will cause the central plasma density to be too high and the etching rate to be faster; By compensating for the edge coil current, the standard deviation of plasma density distribution inside the wafer can be reduced from 8% to 2%.
Dynamic magnetic field adjustment: For different etching patterns (such as high-density and low-density graphic areas), adjust the magnetic field distribution in real time (such as controlling the coil current through a programmable power supply) to avoid local "plasma aggregation" - for example, when etching large-sized grooves, the groove area is prone to attract more ions due to charge accumulation, and dynamically weakening the magnetic field in this area can balance the ion bombardment intensity.
3. Geometric symmetry design of the chamber
The mechanical structure of the chamber should avoid "dead corners" or "asymmetric electric fields" to ensure that the plasma has no disturbance distribution:
Electrode parallelism and symmetry: The parallelism of the upper and lower electrodes (anode/cathode) needs to be controlled within 0.1mm (achieved through precision guides and horizontal calibration devices). If the electrodes are tilted, it will cause differences in local electric field strength (such as edge electric field enhancement), leading to a "fast edge and slow center" etching rate. In addition, the electrode diameter should be slightly larger than the wafer (for example, when the wafer diameter is 300mm, the electrode diameter is 320mm) to reduce "edge electric field attenuation".
To avoid metal contamination and charge accumulation: Inert materials (such as Al ₂ O3 and Y ₂ O3 coatings) should be used on the inner walls and components of the chamber (such as nozzles and liners) to prevent metal atoms from contaminating the sample after plasma sputtering, while reducing uneven charge accumulation on the wall (charge accumulation can distort local electric fields).