Narrowed CD specifications coupled with very tight cycle time requirements have resulted in search for improvement opportunities in CD stability and tuning options for mask fabrication unit processes, including pattern generation, resist development and etch, which may yield narrower scattering band of CD off-target (CDO) of final products. Targeting models are already in productive use at AMTC, accounting for different mask and blank types, clear field, resist type, pattern type and many other parameters. This targeting model is static however, and changes in the CD performance of contributing factors must be adjusted manually when CD drift inevitably occurs. In the past, several approaches to introduce time-based corrections were pursued. Correction of step function of the resulting CDO caused by e.g. resist lot change is the easier task, due to the fact that such factors can be closely analyzed prior to productive use by test, and offset accounting for the individual factor can be introduced. More troubles cause factors, whose effects on CDO is smooth and can be observed as long-term drift in the CDO. The CD drift is frequently of very different origin and effects of several factors are overlapping in time. By measuring the final CD on the products, we can see only the ‘envelope’ of all the effects. To target such factors, we need to identify their root cause and ideally an easy-to-monitor indicator. In this paper we show an analysis approach to identify the most significant and vital indicators to process bias. Analysis of production data covering several manufacturing steps including metrology over more than three years was performed. Using machine learning methods, a “big data” set is reduced, and the most appropriate model is selected using statistical methods. Criteria for selection of factors were significance level in analysis of variance and the distribution of residuals was used for model comparison. Based on these factors a model of the etch contribution to the CD was established, describing the variation of the etch process for a virtual mask with constant clear field, resist sensitivity and absorber composition and thickness. This model is based on the process data collected at the etch process during processing of each mask processed with the same recipe. Monitoring this time trend of the “modelled etch bias” gives very fast feedback about the stability of the etch process and evolution of the etch contribution to CD. This data is used to trigger appropriate corrective actions to further stabilize the manufacturing process.
Continuous shrinking of the semiconductor device dimensions demands steady improvements of the lithographic
resolution on wafer level. These requirements challenge the photomask industry to further improve the mask quality in
all relevant printing characteristics. In this paper topography of the Phase Shift Masks (PSM) was investigated. Effects of
hard mask etch on phase shift uniformity and mask absorber profile were studied. Design of experiments method (DoE)
was used for the process optimization, whereas gas composition, bias power of the hard mask main etch and bias power
of the over-etch were varied. In addition, influence of the over-etch time was examined at the end of the experiment.
Absorber depth uniformity, sidewall angle (SWA), reactive ion etch lag (RIE lag) and through pitch (TP) dependence
were analyzed. Measurements were performed by means of Atomic-force microscopy (AFM) using critical dimension
(CD) mode with a boot-shaped tip. Scanning electron microscope (SEM) cross-section images were prepared to verify
the profile quality. Finally CD analysis was performed to confirm the optimal etch conditions. Significant dependence of
the absorber SWA on hard mask (HM) etch conditions was observed revealing an improvement potential for the mask
absorber profile. It was found that hard mask etch can leave a depth footprint in the absorber layer. Thus, the etch depth
uniformity of hard mask etch is crucial for achieving a uniform phase shift over the active mask area. The optimized hard
mask etch process results in significantly improved mask topography without deterioration of tight CD specifications.