A NEW APPROACH TO HIGH FIDELITY E-BEAM AND ION-BEAM LITHOGRAPHY BASED ON AN INSITU GLOBAL-FIDUCIAL GRID

The distortion-free scan field of an electron-beam or ion-beam lithography system is generally quite small (approximately 10(4) x 10(4) beam addresses) and hence to achieve pattern fidelity over large areas laser-interferometer-controlled stages are employed. Because the laser interferometer monitors the stage, not the electron or ion beam, beam drift of thermal, mechanical, electrostatic, magnetic, or electronic origin is not accounted for, leading to pattern placement error. To overcome this fundamental problem of "dead reckoning" we propose a new approach in which a global-fiducial reference grid, which does not disturb the writing process, is put directly on the substrate. The grid is scanned with sufficiently low areal dose that the subsequent pattern development is not adversely affected. This can be achieved by "sparse sampling" of the grid over the entire scan field in conjunction with phase-locking technqiues in the time domain. In this way one can spatially phase lock the two grids together and thereby ensure pattern placement accuracy. The pattern of interest is then written within the scan field. This method assumes that no drift occurs during the writing of the single field. However, it may also be feasible to do some drift monitoring during the field writing. We consider secondary electrons to be the optimal signal for "seeing" the grid. In addition to providing enhanced pattern integrity (and hence better overlay in 1-to-1 masks), lithography systems based on an in situ global-fiducial grid may prove to be of lower cost than conventional systems since the difficult task of ensuring pattern integrity is thrust upon a computer rather than an advanced electromechanical system.