An electron gun, also known as an electron beam generator, is essential for advanced technologies such as vacuum surface coating, electron beam welding and semiconductor manufacturing. In electron beam physical vapor deposition (EBPVD), a high-powered electron beam evaporates a target material, enabling the subsequent coating process of a substrate. EBPVD is characterized by high deposition rates and allows low substrate temperatures. It is often utilized in roll-to-roll coating of flexible materials with application areas ranging from food packaging and display technologies to photovoltaics.
Numerical setup
To investigate the transport of the electron beam through various stages of the electron gun, the open source plasma simulation software PICLas was used. The simulation includes the acceleration of the electrons to relativistic velocities and their deflection by two magnetic coils. For this purpose, the Particle-in-Cell (PIC) method was coupled with the Monte Carlo Collisions (MCC) method to account for the self-fields of the charged species as well as the ionization processes due to the presence of a neutral background gas. The simulations were conducted using both a 3D model and an axisymmetric setup.
To model the electron generation, a thermionic emission model using the Richardson-Dushman equation including the Shottky effect has been implemented in PICLas. This allows to determine the emission current with the help of the surface temperature. Additionally, the influence of the electric field on the emission current can be investigated. Finally, to reduce the simulation duration a species-specific time stepping technique has been utilized to bridge the gap between the different time scales of the electron movement and ionization processes.
Experimental validation
To validate the simulation results, experimental measurements of the electron beam diameter were performed by an industry partner. A metal mesh was placed at different positions inside the electron gun and the resulting burn-in diameter evaluated. The simulated beam diameter, corresponding to 95% of the total beam energy, is in good agreement with the measured values, confirming the reliability of the numerical model.
Advantages of electron gun simulations
With such investigations, application-related questions can be numerically studied without the risks and costs of physical experiments. Examples of such questions include:
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Effect of assembly tolerances - How does a tilted/misaligned cathode affect the electron beam geometry?
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Influence of background pressure - What happens when the electron beam passes through regions with varying pressure levels?
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Effects of applied potentials and beam current - How do the potential difference and current affect the beam geometry and its energy distribution?
- Influence of the magnetic field - How can the magnetic field be further optimized to achieve a focused electron beam at the target?
