Magnetic Field Stability Analysis of High Voltage Power Supply for Mass Analysis Magnet in High Energy Ion Implanter
High energy ion implanters used in semiconductor manufacturing employ mass analysis magnets to separate ions of different mass to charge ratios, ensuring that only the desired ion species reaches the wafer. The mass analysis relies on the deflection of ions in a magnetic field, with the deflection radius depending on the ion momentum and the magnetic field strength. The high voltage power supply for the magnet coils must provide extremely stable current to maintain constant magnetic field, as field variations cause mass selection errors that degrade implantation quality.
The mass analysis principle in ion implanters exploits the Lorentz force on moving charged particles in a magnetic field. Ions passing through the uniform field region of the analysis magnet follow circular trajectories with radius proportional to the ion momentum and inversely proportional to the magnetic field strength and the ion charge. By placing a slit at the appropriate position, only ions with the desired mass to charge ratio pass through to the subsequent acceleration stage. The resolution, the ability to separate adjacent masses, depends on the magnet design and the stability of the magnetic field.
Magnetic field stability requirements derive from the mass resolution requirements and the consequences of mass selection errors. Field variations shift the position of the mass focal point, potentially allowing unwanted ion species to pass through the slit or reducing the transmission of the desired species. In semiconductor doping applications, contamination by wrong mass ions can degrade device performance or yield. The allowable field variation depends on the mass separation between the desired and contaminant species and the slit geometry.
High voltage power supplies for magnet coils must provide current stability that translates to the required field stability. The magnetic field is proportional to the coil current for properly designed magnets operating below saturation. Current ripple at power line frequencies or switching frequencies causes field modulation that can affect the mass resolution. Long term current drift causes field drift that shifts the mass calibration. The power supply specifications must ensure that current variations remain within the stability budget.
Temperature effects on the magnet and power supply contribute to field instability. Coil resistance increases with temperature, affecting the current for a given voltage in voltage regulated supplies. Thermal expansion of the magnet structure changes the geometry and thus the field for a given current. Temperature variations in the power supply components affect the output characteristics. Temperature control of the magnet and the power supply, or temperature compensation circuits, maintain stability over the operating temperature range.
Magnet hysteresis introduces history dependence to the field current relationship. Iron dominated magnets exhibit hysteresis where the field for a given current depends on the magnetization history. Cycling the current through a hysteresis loop before setting the operating point can establish a reproducible initial state. The power supply may include degaussing cycles that systematically reduce the current to demagnetize the iron before setting the desired field.
Current regulation approaches for magnet supplies include voltage regulation with current feedback, direct current regulation, and regulation based on magnetic field measurement. Voltage regulation is simple but requires compensation for resistance changes with temperature. Current regulation maintains constant current regardless of resistance changes, providing better stability against temperature variations. Field regulation using a magnetic field sensor in the magnet gap directly controls the quantity of interest but requires sensor placement and calibration.
Power supply topology for magnet current supplies typically uses switching converters for efficiency at high power levels. The switching frequency affects the output ripple and the response bandwidth. Higher switching frequencies enable smaller filter components and faster response but may increase losses. Multi phase interleaved converters reduce the ripple through phase cancellation. The converter design must balance efficiency, ripple, and response characteristics for the application.
Protection systems for magnet power supplies address both equipment protection and process protection. Overcurrent protection prevents damage to the coils from excessive heating. Overvoltage protection addresses transients from inductive kick during current changes. Ground fault detection identifies insulation failures in the magnet or cabling. Fast shutdown capability enables rapid field reduction if required by process conditions or safety interlocks.
Calibration and verification of the mass analysis system relate the power supply setpoint to the actual mass selection. Mass calibration using known ion species establishes the relationship between current and mass. Periodic verification confirms that the calibration remains valid. Diagnostic measurements of the beam profile at the mass slit can indicate the resolution and alignment. These measurements support the quality assurance requirements for semiconductor manufacturing equipment.
