Characterization of CCD sensor for actinic mask blank inspection

摘要

Back-illuminated charge-coupled devices (BI-CCD) have been characterized for the application of actinic EUVL mask blank inspection. Point spread function (PSF) of a BI-CCD is analyzed by extracting the EUV-emulating events from photon counting images of 55Fe exposure experiments. The spread of the PSF is found to be reduced by applying higher voltage to extend the depletion layer, which is consistent with the physical model of the electron diffusion in the backside region. High speed capability of the most promising BI-CCD is also tested to investigate its applicability to production-worthy high throughput inspection tools. It has been demonstrated that a clear image can be obtained at 5.6MHz clock speed which translates into the throughput of 2 hours per blank using 26x imaging optics. The readout noise at this clock speed, however, has turned out to be extremely degraded than is required to keep the required sensitivity. Potential approaches to overcome sensitivity degradation caused by the increase in the readout noise are discussed.