During the last years, new microscope applications require an increased resolution which enforces the development of new state of the art high NA immersion objectives. With the introduction of the 4Pi confocal fluorescence microscope, the increase of the numerical aperture from NA=1.4 to NA=1.46 makes sense, although the gain of lateral resolution is quite small. On the other hand, for inspection and metrology in the semiconductor industry the continuously decreasing structures need the highest possible resolution, which can be achieved with high NA water immersion objectives working in the DUV wavelength range. Building this kind of objectives requires special measuring and testing technologies and a manufacturing precision which has never been realized before in series production.
When looking into even modern textbooks on optical design and engineering one can get the impression that everything has already been said about microscope optics. Also at optical conferences microscope optics is at most a marginal topic. Because of this only insiders are aware about the exciting challenges and really amazing achievements in today's state-of-the-art microscope optics. To give an example, this paper will focus on ultra-high resolution DUV microscope objectives for semiconductor inspection and metrology with feature size (half pitch) resolution down to about 60 nm. To meet the performance requirements of such objectives all aspects of optical design and tolerancing, optical coating design, optical production, assembly and image performance assessment have to be matched perfectly to each other at the highest technological level.
The increased requirements on reticles for the 65nm technology node with respect to CD homogeneity and CD mean to target requirements call for a metrology system with adequate measurement performance. We report on the new water immersion technique and the system concept of the worlds first optical CD metrology system based on this technology. The core of it is a new DUV immersion objective with a NA of 1.2, using illumination at a wavelength of 248nm. The largest challenge of the water immersion technology was the fluid handling. The key compo-nents, a water injection and removal unit, developed by MueTec, solve this issue. To avoid contaminations the purified DI water is micro-filtered. An environmental chamber guarantees extremely stable measurement conditions. The advantages of optical CD measurements in transmitted light compared to CD-SEM is shown. With this system, already installed, excellent results for short- and longterm repeatability for both linewidth and contact measurements were achieved on COG, KrF HT and ArF HT masks. The linearity range of the system is extended down to 220nm. A comparison of CD measurements between the different tool generations such as the Leica LWM250/270 DUV at 248nm with a NA of 0.9 is shown. An outlook on the future potentials of optical mask CD metrology finalises this report.
This paper addresses the possibilities and recent achievements in increasing significantly the resolution of optical microscopy for wafer and mask inspection and metrology. DUV microscopes operating at 248 nm wavelength already offer a feature size resolution down to 0.08 micrometers . Photon tunneling microscopy (solid immersion optics) allows to apply the higher resolution of immersion optics without bringing the immersion into contact with the specimen. First results are shown. A special illumination mode in laser confocal microscopy, the so called doughnut illumination, appears to have the potential for increasing the resolution by about 30% compared to classical laser confocal microscopy. It is shown that in the combination of these three methods an ultimate feature size resolution of about 25nm may be achieved- at least theoretically. The future will show if what seems to be physically feasible can be transformed to technical solutions.