The distribution of residual gas in an ion-implant chamber

Using a new technique in the field of finite-element analysis (FEA), the high-vacuum pressure distribution of residual gases in an implant chamber is calculated. The technique assumes that the gas molecules, which obey the laws of molecular flow, do not interact with each other but only with the inside surfaces of the implant chamber. The steady-state distribution is dependent on the outgassing load within the chamber, the gas species, the pumping speed and location of the vacuum pumps, and the geometry of the chamber. The effect of geometry on the distribution also includes “shadowing” of the disk inside the chamber. For a given wafer-photoresist outgassing load, the pressure distribution of each constituent gas is determined separately. The pressure distribution is shown graphically as a contour map inside the chamber. The total gas distribution is the sum of the constituent-gas distributions. Also, an equivalent “beam impact ratio” is introduced and defined as the total number of gas molecules through which an ion beam must penetrate to reach its wafer target. The barrier is calculated by integrating the molecular density along a line between the beam-entrance aperture and the wafer. The assumption of a cosine, or “lambertian” distribution for outgassing from the wafers is used in the calculations. The influences of pumping characteristics, chamber geometry, disk-tilt angle, and outgassing load on the barrier are determined. The goal of this study is to determine the distribution of gas inside the chamber during an implant cycle