Design and Development of a Multiaxial Astigmatic Displacement Measuring System

In this dissertation, a novel non-contact multiaxial astigmatic displacement measuring system (MADMS) is designed and developed using the astigmatism as the measuring principle for the translational displacement, the angle, and their variations of a measured surface simultaneously. The pickup head of a commercial digital versatile disk (DVD) read only memory (ROM) drive can be used directly as an optical path mechanism in the above mentioned MADMS, which can measure the translational and angular displacements accurately and simultaneously. The total linear detection range and the maximum measurement bandwidth of the MADMS are 6 um and 80MHz, respectively. The resolution of the translational displacement measurement is in sub-angstrom scale. For an operating frequency of 700 kHz, the noise floors of the translational and angular signals are below 0.8 pm/Hz1/2 and 0.4urad/ Hz1/2, respectively.
The MADMS can monitor the translational and two orthogonal angular displacements of a micro fabricated cantilever in atomic force microscopy (AFM). All the three, contact non-contact and tapping, modes can resolve the single atomic steps of the graphite surface, which indicates that atomic resolution is achievable with the MADMS. The thermal noise spectra of the AFM probe can be clearly measured as well. Furthermore, the accuracy of scanning probe microscopy (SPM) depends not only on the measurement system itself, but also by the accuracy of the signal processing, which further depends on the physical and geometrical characteristics of the probe.
The structure of the MADMS is compact and stable. Besides the measurements through AFM probes, the MADMS can be operated in profilometer mode. The CD surface and the CCD microlens are measured by this mode. The maximum scanning speed can reach up to 3.84×106 um/s theoretically, almost one million times faster than that of a commercial SPM system.
The MADMS has great potential for future development, the expansibility and the accuracy can evolve with the performance of future pickup head. From the DVD pickup head to higher resolution one, such as the pickup head of the Blu-ray drive or high- definition (HD-DVD), can be integrated into the MADMS as well.