عنوان مقاله [English]
A distance sensing apparatus for use in autofocus systems of laser lithography is developed. A nozzle is placed a small distance away from the photo-resist and the inside gas pressure is used as a measure of clearance. A differential circuit is chosen which consists of a measurement path of gas and a bleed line, which is used as a reference. The flow pattern of the main part, i.e. the nozzle, is explored via computational fluid dynamics, and it is shown that pressure loss in the cylindrical space between the nozzle and the sheet dominates. Moreover, available empirical correlation is shown to be valid, even in the presence of an objective lens inside the nozzle.The governing equations f ll omponents of the circuit, including the nozzle and the valves, are solved simultaneously, to yield operating pressures and flowrates. This provides a mathematical design tool that helps examine the influence of design parameters, particularly on accuracy and finding optimum values. Sensitivity is estimated by the derivative of differential pressure, with respect to clearance. It is predicted that higher pressure drop across the valve, and, to a lesser extent, the larger nozzle outer diameter, will result in improved sensitivity, while optimum inner nozzle diameter and the design point clearance are dependent on the level of drop in valve pressure. So, along with more restrictive valves, these two parameters should be chosen smaller, if higher accuracies are to be met. Provisions are also made for inlet pressure fluctuations and the gas reservoir.Experiments are conducted on a single path of the circuit and also on a constant-flow nozzle. Nozzle pressure vs. distance was recorded, showing the same trend as predicted by simulations. Experiments demonstrate that sensitivity is improved at closer distances and with more gas consumption rates. Magnification of 20mm per micron is achieved at a distance range of 50-100 microns. Results were stable and repeatable. Feasibility of the application of this low-flow device to working distance adjustment in lithography was proven.