Measurement Systems For Image Placement in SCALPEL Projection Electron Lithography

The image placement requirements for lithography of the 100 nm generation of integrated circuits and beyond are unprecedented for an e-beam lithography system (<10 nm). In lieu of an aerial image monitor for projected e-beam images, indirect measurements are used that combine optical interferometry and detected backscattered electrons (BSE). The locations of the wafer and mask stages are monitored using laser interferometers and the signals are fed back to an electron beam deflector to keep the electron image of the mask stable at the wafer. Calibration and maintenance of this error correction system is done by the measurement of BSE that are generated by scanning the electron image of a mask mark over a corresponding wafer stage mark. Initial results using a SCALPEL proof of lithography stepper gave pattern stitching accuracy of 50 nm (3 sigma). Similar mark detection is required to monitor the image focus, astigmatism, magnification and rotation. The measurements from fiducial marks on a wafer will also contain the effects of processing and dynamic effects such as wafer heating due to e-beam irradiation. Solutions for these measurement problems will be discussed using models of a SCALPEL system operating under high wafer throughput scenarios. This work was partially supported by DARPA under contract MDA972-95-C-0013.