Beam Focusing and Deflection Coordinated Control Strategy of High Voltage Power Supply for Electron Beam System
Electron beam systems are used in diverse applications including welding, additive manufacturing, microscopy, and lithography. The electron beam must be precisely focused and positioned for these applications. The high voltage power supplies that bias the focusing and deflection elements must be coordinated for accurate beam control. A coordinated control strategy ensures that the focusing and deflection work together to maintain optimal beam characteristics.
Electron beam systems generate electrons from a cathode, accelerate them to high energy, and focus them into a fine probe using electromagnetic or electrostatic lenses. The beam is then deflected to the desired position on the workpiece. The beam characteristics including spot size, current density, and position must be precisely controlled for the application requirements.
Focusing uses electron lenses to converge the beam to a small spot. Electromagnetic lenses use current carrying coils to generate magnetic fields that bend the electron trajectories. Electrostatic lenses use electrodes at different potentials to create electric fields that focus the beam. The lens strength determines the focal length and the spot size at the workpiece.
Deflection moves the beam position on the workpiece. Electromagnetic deflection uses coils to generate magnetic fields perpendicular to the beam axis. Electrostatic deflection uses electrode pairs to create transverse electric fields. The deflection sensitivity determines how much the beam moves for a given deflection signal. The deflection must be linear and accurate across the entire scan field.
The high voltage power supplies for focusing and deflection must be stable and precise. The focusing lens supply determines the lens strength and the spot size. Variations in the lens supply cause the spot size to fluctuate, affecting the process. The deflection supplies determine the beam position. Variations in the deflection supplies cause position errors.
Coordinated control addresses the interactions between focusing and deflection. The beam characteristics at the deflection point depend on the focusing. Changes in focus affect the deflection sensitivity and linearity. Changes in deflection position can affect the effective focus due to lens aberrations. The control strategy must account for these interactions.
Focus tracking adjusts the focus as the beam is deflected across the workpiece. Electromagnetic lenses have aberrations that cause the focal length to vary with the deflection angle. Without correction, the spot size would increase at the edges of the scan field. Dynamic focus correction adjusts the lens current based on the deflection position to maintain constant spot size across the field.
Rotation correction addresses the rotation of the deflection coordinate system with focus changes. In electromagnetic systems, the magnetic field of the lens rotates the beam as it focuses. This rotation causes the deflection coordinates to rotate relative to the workpiece. The rotation angle depends on the lens strength. The control system must compensate for this rotation to maintain correct beam positioning.
Hysteresis compensation addresses the magnetic hysteresis in electromagnetic lenses and deflection coils. The magnetic field depends on the history of the current, not just the instantaneous value. This hysteresis causes position errors that depend on the scan history. Compensation algorithms model the hysteresis and apply corrections to achieve accurate positioning.
Calibration establishes the relationship between the control signals and the beam characteristics. Focus calibration determines the lens current for optimal focus at different working distances. Deflection calibration measures the beam position for known deflection signals, establishing the deflection sensitivity and any nonlinearity. The calibration data enables accurate control based on the desired beam parameters.
Real time control updates the focusing and deflection based on the process requirements. For scanning applications, the control system generates the deflection waveforms and the corresponding focus corrections at high speed. The update rate must be sufficient for the scanning speed and the required accuracy. Digital signal processors or field programmable gate arrays provide the computational power for real time coordinated control.
Feedback from beam monitoring enables closed loop control. Beam current sensors measure the total beam current. Beam position sensors detect the beam position for alignment and calibration. Backscattered electron detectors can provide information about the beam interaction with the workpiece. This feedback enables correction of drift and disturbances during operation.
