Jenspeter Rau

EUV mask development: Material and process

Extreme ultraviolet lithography (EUVL) is one of the most promising technologies for wafer feature sizes of below 50nm. The illumination wavelength will be approximately 13.5nm and consequently no transmissive optics can be used for this soft X-ray light. For several years intensive research work has been performed in different programs mainly through EUV-LLC, ASET and PREUVE. This has resulted in providing solutions for the most critical tasks of EUVL - powerful sources, defect free mask blanks and environmentally stable optics of high reflectivity. During the development with EUV-LLC an engineering test stand for illumination has been built which will be a powerful tool for the development for EUVL masks. We have studied the patterning of a EUVL mask for process development and repair tests. The material was a standard Cr absorber (as used in production) and a SiO2 buffer layer. The process investigation was focused on the dry etch stop behaviour of the etch processes and also on cleaning issues. The mask concept favoured today for EUVL masks is very similar to the masks used in production; consequently most work is performed on Cr as the absorber material and SiO2 as the buffer material. From results presented in recent years we can surmise that Cr and TaN are at present the most promising candidates as absorber materials. However it is also known that it will be very difficult to develop an etch-bias free process for Cr. In this paper we shall present our results detailing the etch properties of Ta and TaN as an absorber material. TaN is shown to be a promising absorber material. In addition, the impact of mask properties on placement and bow has been investigated with finite element calculations.

Optimized processes and absorber-stack materials for EUV masks

Currently, EUV lithography targets for sub-50 nm features. These very small feature sizes are used for reflective illumination and impose great challenges to the mask maker since they do not allow a simple downscaling of existing technologies. New material combinations for absorber and buffer layer of EUV masks have to be evaluated and fundamental material limits have to be overcome. We report on optimized absorber-stack materials and compare in particular the performance of chrome and tantalum nitride for such small nodes. Tantalum nitride shows similar or even better properties than standard chrome, above all with respect to etch bias. Further investigations have to be done but this material is a promising candidate for feature sizes in the sub-50 nm range.

Development of a plasma etch process for TaN absorber patterning on EUV masks

EUV mask technology poses many new challenges on mask manufacturing processes. One crucial manufacturing step is the patterning of the EUV absorber. Although in the first concepts a Chromium film is used as absorber, increasing demands for shrinking feature sizes will run Chromium out of steam. Due to the necessary oxygen content of the chromium etch plasma and the isotropic etch mechanism for chromium an etch bias of several 10 nm occurs. This results in limitations for the minimal feature size, for which reason a new absorber material has to be developed. The most promising candidate is Tantalum Nitride TaN, which in contrast to the isotropic Cr-etch process, gives the possibility of applying a more anisotropic etch utilizing higher ion energies and sidewall passivation. In this work a plasma etch process for TaN masked with positive CAR resist was developed on masks including a SiO2 buffer layer. Before running the experiments for process characterization, an endpoint detection solution by OES for very small open areas was developed utilizing principal components analysis (PCA). Additionally, an experimental matrix was set up varying bias power, source power and pressure. The DoE experiments were analyzed with respect to etch selectivities, etch bias, etch polymer formation, sidewall angle, iso-dense bias and linearity. After characterisation of the experimental results, optimized process conditions are discussed. We show that this process is capable of resolving feature sizes below 100 nm.

Mask CD characterization with EUV reflectometry at the electron storage ring BESSY II

CD metrology requirements have increased dramatically within the last years. For the coming technology generations, it is not clear which CD measurement method will be standard for mask manufacturing. An interesting approach is to use the diffracted signal of periodic mask patterns for determination of CD. For wafer CD measurement, CD scatterometry tools using visible or UV wavelengths are already commercially available. For this experiment, diffracted EUV light was used. Dense lines of pitches 1:1, 2:1 and 5 :1 and nominal CDs of 150 nm, 200 nm, 300 nm, 400 nm and 500nm have been illuminated with EUV light of ?= 13.35 nm at the BESSY II storage ring in Berlin. The reflected signal has been collected with a movable detector in a range of -1 ° to 200 relative to the specular reflection. With the angular position of the peak, the pitch can be calculated. The CD, however, is related to the intensity of the peaks. Several effects as mask topography and measurement uncertainties are discussed. The results are compared to CD-SEM measurements of the same patterns.