An efficient chamber detector for detection of secondary electrons (SE) was designed and tested in Raith’s ionLINE ion lithography tool. The design was based on 3D simulations of electric fields generated by the voltages and the electrode structures of all the elements affecting SE. A realistic physical model for the SE was used in the 3D model to calculate and optimize the SE collection efficiency (CE).
A noise “cascade model” was developed to identify the influence on the SNR of each signal transformation stage from the primary beam to the PMT output.
This model was used to derive the CE from the experimental images SNR, found to closely match the simulated values.
The concept of ”Acceptance Map” was employed for relating the initial directions of the SE to their hit location in the chamber, such as pole piece or detector. It was found that the FIB objective lens attracts most of the vertically emitted SE (at small angles to the normal to the sample) such as SE from Vias or trenches, preventing them from reaching the detector.
Introducing a weak magnetic field under the FIB pole piece, these electrons can be directed towards the detector. Simulations of the electron motion under the combined electric and magnetic fields enables to optimize the collection efficiency by the suitable choice of both fields. Such add-on units, that consists of two small permanent magnets were developed and implemented in the ionLINE tool showing promising preliminary results.
