Focus Detection Method Based On Superposed Grating Moiré In Nano-lithgraphy

In the information era, the intergrated circuit technology, which is used for chip manufacturing, is heading towards narrower inter-connection lines, higher density, and larger scale, and these trends greatly inspire the development of lithography tools on the behalf of the micro-electronics equipments. As one of the critical measurement technologies in lithography processes, focus detection is the precondition process to focus and level the wafer into the depth of focus(DOF) of the lithographic objective. Generally, the accuracy of focus detection must achieve 1/10 of the DOF, i.e. at nano-meters level. In this situation, traditional focus detection technologies that adopted light-intensity-based slot/pinhole arraies encounter challengees to satisfy that high-precision. The requirement for new kinds of focus detection technologies is urgent. Hence, this dissertation aims to explore a superposed grating moiré based focus detection method, which modulates the defocus amount into the spatial phase of the moiré fringe. Combining the advantages of the image processing and interferomitric measurement, the moiré-based focus detection method could achieve wafer metrology at nano-meters level. At the same time, this method is insensitive to the photoresist coating, lighting source, and the objective ratio showing higher anti-interference and technological adaptability performance.Focus detection is to precisely determine the vertical and tilt defocus amounts of the wafer, meanwhile achieves accuracy at nano-meters level and measurement range during dozens of micro-meters. Regarding above demands, this dissertation firstly explores the light field, where moiré fringes form from, behind the superposed grating marks using scalar diffraction theory. Then, the focus grating marks are designed, and four channel model of spatial image focus detection method is built, also the coarse-fine focus detection scheme is adopted to enlarge the measurement range. Further, the vertical and tilt defocus amounts, which leads to transverse moiré shift and moiré twist respectively, are detailed studied. To quantitatively calculate those two kinds of defocus amount, the phase-demodulation-based method is introduced, and the influence of objective ratio variation is also considered in the calculation. What′s more, because of the structure complexity of the coarse-fine scheme, an improved method that based on dual-frequency- moiré is put forward to enlarge the measurement range with convenience and simplicity. Finally, the focus detection experimental platform is constructed to verify the fisibility and accuracy of proposed methods. Results indicate that both the coarse-fine method and dual-frequency-moiré could successfully calculate the defocus amount within dozens of micro-meters; and overally, the detection accuracy of vertical defocus amount is less than ±10nm; and the detection accuracy of tilt defocus amount attains 0.001° level. All these performanced results jointly demonstrate the rationality of proposed methods.