Abstract
While the industrial implementation of extreme ultraviolet lithography for upcoming technology nodes is becoming ever more realistic, a number of challenges have yet to be overcome. Among them is the need for actinic mask inspection. We report on reflective-mode lensless imaging of a patterned multi-layer mask sample at extreme ultraviolet wavelength that provides a finely structured defect map of the sample under test. Here, we present the imaging results obtained using ptychography in reflection mode at 6° angle of incidence from the surface normal and 13.5 nm wavelength. Moreover, an extended version of the difference map algorithm is employed that substantially enhances the reconstruction quality by taking into account both long and short-term variations of the incident illumination.
Highlights
Considerable effort has been spent on the development of extreme ultraviolet (EUV) lithography to make the transition from deep ultraviolet lithography in upcoming technology nodes [1] and it is believed that the technology will be ready for the 7 nm node
We have recently presented defect maps, both from die-to-die and die-to-database comparison, showing a 50 × 200 nm defect with a high signal-to-noise ratio [20], underlining the feasibility of using ptychography with the difference map algorithm adapted to an EUV reflective-mode setup for reliable EUV defect inspection
We have shown that ptychography can be used to image absorber patterns on EUV photomask samples in reflection mode with negligible artifacts as well as high resolution and thereby demonstrated the feasibility of coherent diffraction imaging (CDI) for the inspection of defects on EUV photomasks
Summary
Considerable effort has been spent on the development of extreme ultraviolet (EUV) lithography to make the transition from deep ultraviolet lithography in upcoming technology nodes [1] and it is believed that the technology will be ready for the 7 nm node. A method for the reliable detection of mask defects remains a challenge [2, 3] In this context, a defect is defined as any structure in the fabricated mask that will lead to a fault when copied to the wafer. A defect is defined as any structure in the fabricated mask that will lead to a fault when copied to the wafer Existing metrology tools such as scanning electron microscopy, are reliable and well established but due to the use of electrons or photons at wavelengths different from the EUV design wavelength, the aerial image will differ from that of the scanner and could include non-printable defects while missing others that could lead to device failure [4]. To avoid printing defects to the exposed wafers and to maximize yield, EUV metrology methods are essential
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