Focal plane flatness of an electrostatic lens
F.T. Overes (TU Delft - Mechanical Engineering)
JFC van Gurp – Mentor (TU Delft - Micro and Nano Engineering)
J. W. Spronck – Graduation committee member (TU Delft - Mechatronic Systems Design)
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Abstract
Holistic lithography uses feedback from a chip to improve the production process for the following batches of chips. However, as the features on a chip are on a nanometer scale, acquiring feedback is not simple. The features are too small for conventional light based metrology systems, therefore, scanning electron microscopes (SEM’s) are used.
As a SEM uses one single beam it is too slow to implement in the lithography production process. To improve the speed multibeam SEM’s have been designed. These microscopes use the same operating principle but with multiple beams simultaneously.
Each beam has its own focal point, which together form the focal plane. To acquire a sharp image the focal plane needs to very precisely align with the sample that has to be measured. To achieve this the focal plane has to be very flat, which requires a very precise electrostatic lens.
In this thesis the flatness tolerances are evaluated and determined whether they are feasible. This is evaluated by calculating the errors with FEM software and by experimentally testing the remaining errors.