In-Die Phase Measurement Tool Mimics Scanner

The use of phase-shift masks (PSMs) with 193 nm scanners of high- and hyper-numerical apertures (NA) with special illumination has enabled the progression of optical lithography's resolution limits to the 32 nm node. The downside is that mask complexity increases exponentially alongside the urgent requirement for accurate phase control to reach requisite high yields.

The phase in the image plane of a scanner is strongly affected by its NA, the mask's pitch and 3-D mask effects, particularly for feature sizes that come close to the resolution limit. For accurate printing of a PSM for the 45 and 32 nm nodes, it becomes necessary to measure the scanner-relevant phase in production features if adequate yield is to be achieved. However, control requires precise and accurate measurement and, until now, phase has been measured using interferometer-based metrology tools. A problem with this is that these platforms are limited to the evaluation of phase by using large reference features that exceed production features by an order of magnitude. High-resolution atomic force microscopy (AFM) tools have also been brought to bear on the problem and, although capable of measuring etch depth in production-relevant features, these cannot reveal 3-D mask effects. Both methods do not capture diffraction limitations by NA and pitch, as well as rigorous 3-D mask effects.

Working with Intel (Santa Clara, Calif.), the Carl Zeiss Semiconductor Metrology Systems Division (Jena, Germany) researched the requirements for an optical phase measurement tool that would have the capability to extend process control from large CD test features to in-die phase-shifting features with high spatial resolution. It was determined that such a tool would have to be designed to mimic the litho platform's optical setup and acquire the phase information generated from feature sizes in the wavelength's length.

The result is an optical phase measurement tool, Phame, which can measure scanner-relevant phase for all PSMs in-die under scanner-relevant settings, accounting for polarization. It covers the capability of existing tools measuring large reference features, as well as that of measuring production features capturing real-world mask effects. The platform's optical beam path is comparable with that of an immersion scanner with a 1.6 NA. Its 193 nm laser is combined with a low sigma unit that coherently illuminates the mask, which is handled face down. On- or off-axis illumination can be used depending on the PSM type.

The partial coherent illumination settings of a scanner can be sampled in consecutive measurements of adjustable intervals, allowing phase control under scanner-relevant illumination settings. The tool's precision imaging optics with a 0.4 NA (1.6 NA scanner equivalent) makes the system compatible with 193 nm immersion scanners down to the 32 nm node. Phase information is obtained through phase manipulation and algorithms, and the CCD camera occupies what in an actual scanner would be the wafer's position. Besides in-die phase value, the tool also measures in-die transmission.

The Phame platform is designed to mimic a lithography tool’s setup, and provide measurement capabilities to optimize OPC processes and improve masks. (Source: Carl Zeiss)

The system provides three different measurement modes: manual, histogram analysis and defined area. Intensity and phase images are acquired during the measurement, while the phase profile can be manually generated by choosing a slice along the phase image showing the corresponding phase values over it. The histogram analysis then averages phase values over the complete measurement area. This option is used for phase control after etch and cleaning of the PSM. Additionally, other areas can be defined or automatically set by the software within the phase image, and averaged phase differences can be evaluated. A correlation between defined areas can be done for repair verification or for the evaluation of optical proximity effects.

The platform can measure all types of PSMs. It has demonstrated a phase accuracy of <1°, static phase reproducibility between 0.15° and 0.3° for small production features, and <0.2° for large reference features.

While this measurement system is primarily intended for mask house process control and the optimization of process windows to enhance yield, it should have extensive application in the R&D arena to optimize optical proximity correction (OPC) processes while improving masks and shortening the time for design optimization.

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