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为用电子束实行大面积曝光,每次扫描场通常都限制在2毫米×2毫米内,而每次扫描完成以后基片都是通过机械方式步进到光轴下的位置,衬底位置须精确对准以防止相邻的场产生不能容许的误差。通常的对准技术是利用由曝光抗蚀剂用的电子束得来的信号探测半导体片上的一组对准标记,然后将对准标记或电子束的位置调定到对准扫描场所要求的精度。 对准标记在基片上占据着有用的面积,电子束在搜索对准标记的位置时,会使标记周围的一部分地方曝光,因而使这些区域变得无用了。其次,某些大面积器件结构的几何图形较大,因此要求在器件面积内、完全消除对准标记。最后对涂有一层抗蚀剂的对准标记进行定位是很困难的,因为这些标记的边界显不出很强的衬度。 为了克服上述缺点,提出了一种新的曝光方法。其原理是在搜索操作时使电子束减速以至电子的能量很低,只能使抗蚀剂极薄的表面层曝光。一旦探到要曝光的面积,就使电子束的能量恢复到抗蚀剂最佳曝光所要求的全值。这样,在搜索操作过程中使没有抗蚀剂的部分曝光,就完全避免了有用面积的浪费。且在不容有对准标记的地方,第一次扫描场中曝光过的图形边缘可在下一次扫描场中用作对准。这就要求对第一次图形进行部分显影,且过程要慢。但这种方法仅适用于一些特殊器件。
In order to perform large-area exposure with an electron beam, each field is usually limited to 2 mm × 2 mm. After each scan, the substrate is mechanically stepped to the position under the optical axis. The substrate position Precise alignment prevents unacceptable errors in adjacent fields. Conventional alignment techniques utilize a signal from an electron beam for exposing a resist to detect a set of alignment marks on a semiconductor wafer and then set the alignment mark or the position of the electron beam to the accuracy required for alignment of the scanning area . The alignment mark occupies a useful area on the substrate and the electron beam, when it searches for alignment marks, exposes a portion of the area around the mark, rendering these areas useless. Second, the geometry of some large area device structures is large, requiring the alignment mark to be completely eliminated within the device area. Finally, it is difficult to locate alignment marks that are coated with a layer of resist because the boundaries of these marks do not show a strong contrast. In order to overcome the above shortcomings, a new exposure method is proposed. The principle is that the electron beam is decelerated during the search operation so that the electron energy is very low, which can only expose the extremely thin surface layer of the resist. Once the area to be exposed is explored, the energy of the electron beam is restored to the full value required for optimal exposure of the resist. In this way, the portions without resist are exposed during the search operation, completely avoiding the waste of useful area. And where no alignment mark is acceptable, the exposed graphics edges in the first field can be used as an alignment in the next field. This requires a partial development of the first graphics, and the process is slow. However, this method only applies to some special devices.