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LINAC simulation for high energy ion implantation

RF linear accelerators (LINACs) are routinely used to accelerate ions to few hundreds of MeV in high energy physics accelerators. When this technology is applied to high energy ion implantation for semiconductor device fabrication, parameters such as high transmission, low energy spread, and small beam emittance must be preserved over a wide range of energies and masses. We have modified a particle tracking code, PARMELA (Phase and Radial Motion in Electron Linear Accelerators), to allow the investigation and optimization of the beam characteristics of Eaton's rf LINAC. We present the main principles of this simulation and show several examples of beam propagation studies (e,g., effects of beam bunching, beam emittance, mechanical alignment, and other factors on beam transmission), In a typical simulation of the accelerator in Eaten's NVGSD/HE ion implanter, which has 10 individual rf drift tube resonators, we have found that ion beam bunching occurs in the first few rf drift tubes. Bunched ions are accelerated and transported through the accelerator. For the other ions, most of them are lost because they strike the walls of rf drift tubes and focusing electrostatic quadrupoles along the accelerator. These transmission loss mechanisms increase monotonically with increased beam current, which eventually limits the maximum output current.