German E. Rylov

RELAX: resolution enhancement by laser-spectrum adjusted exposure

In this work, we demonstrate a resolution enhancement technique for DUV lithography in which the light source spectrum is modified in order to improve the imaging performance of given device patterns. With this technique, termed RELAX, the imaging depth of focus (DOF) can be improved significantly for contact holes, and potentially line-space patterns. The improvement in the DOF comes at the expense of modest deterioration of other process performance metrics, such as exposure latitude and exposure bias, due to reduced image contrast at best focus. Compared to the FLEX-based techniques, RELAX allows a continuum of tunable spectral conditions without the drawback of multiple exposure passes, which is especially critical for step-and-scan lithography. Spectrum modification is accomplished by replacing the line narrowing and wavemeter modules of the excimer laser light source with RELAX-enabled modules. Direct wavefront modification of the laser output has been demonstrated to provide the optimum method for producing a double peak spectrum, which simulation has shown to produce the maximum DOF benefit. Results from imaging experiments of attenuated-PSM contact structures exposed using 248nm dipole illumination showed DOF improvements of up to 70% with a double peak separation of about 2pm. Lateral chromatic effects at this separation were negligible. These results agreed well with previous double exposure experiments1 and simulations of some of the design structures. The process improvements were obtained without a need for re-biasing of the mask structures, although a dose adjustment was required.

Advanced VUV spectrometer for F2 laser metrology

The advent of 157 nm F2 lasers in lithographic application implied new challenges in spectral metrology. The approaches for the lithographic imaging system, that have been suggested so far, differ in the requirements of the spectral bandwidth of the laser. However, even designs with less stringent demands will require high-resolution spectral metrology in order to enable comprehensible spectral bandwidth and purity measurements or specifications. Ideally, narrowband calibration sources in the VUV range should be used to precisely determine the instrument function of the spectrometer, enabling correct spectral de-convolution. However, most schemes for generation of appropriate light sources are rather complex and thus expensive. Stability and lifetime of solid state sources, i.e. nonlinear optical devices, are expected to be not satisfying too. The principal approach for a spectrometer design should be to increase the inherent spectral resolution of the instrument above the required specification limits of the laser systems under investigation, avoiding or at least significantly reducing the necessity of spectral de-convolution. Following this path, the optical layout of an existing Echelle grating based spectrometer has been investigated and re-designed. Collimation and imaging quality of the spectrometer could be considerably improved with the implementation of an aspheric focusing mirror.