Extreme ultraviolet (EUV) microscopy is an important tool for the investigation of the performance of EUV
masks, for detecting the presence and the characteristics of defects, and for evaluating the effectiveness of defect repair
techniques. Aerial image measurement bypasses the difficulties inherent to photoresist imaging and enables high data
collection speed and flexibility. It provides reliable and quick feedback for the development of masks and lithography
system modeling methods.
We operate the SEMATECH Berkeley Actinic Inspection Tool (AIT), a EUV microscope installed at the
Advanced Light Source at Lawrence Berkeley National Laboratory. The AIT is equipped with several highmagnification
Fresnel zoneplate lenses, with various numerical aperture values, that enable it image the reflective mask
surface with various resolution and magnification settings. Although the AIT has undergone significant recent
improvements in terms of imaging resolution and illumination uniformity, there is still room for improvement.
In the AIT, an off-axis zoneplate lens collects the light coming from the sample and an image of the sample is
projected onto an EUV-sensitive CCD camera. The simplicity of the optical system is particularly helpful considering
that the AIT alignment has to be performed every time that a sample or a zoneplate is replaced. The alignment is
sensitive to several parameters such as the lens position and orientation, the illumination direction and the sample
characteristics. Since the AIT works in high vacuum, there is no direct access to the optics or to the sample during the
alignment and the measurements. For all these reasons the alignment procedures and feedback can be complex, and in
some cases can reduce the overall data throughput of the system. In this paper we review the main strategies and
procedures that have been developed for quick and reliable alignments, and we describe the performance improvements
we have achieved, in terms of aberration magnitude reduction.