ArF immersion lithography has opened the road towards increased optical resolution at the 193nm wavelength. Consequently, keeping the same 4X optical demagnification factor, the dimensions on the mask scale down to sub-wavelength values when we enter the 45nm node. At such dimensions, mask topography, mask type and materials as well as the polarization state of the light will influence the diffraction spectrum of a layout. As a result the image from high NA lithographic systems depends on the polarization state and intensities of the interfering orders. In general, with smaller features on the mask stronger polarization changes occur. Apart from the polarization changes in diffraction orders the total intensity in a diffraction order is also different from that predicted by standard scalar-Kirchhoff diffraction approximation used in present OPC packages. The difference in intensities of diffraction orders due to different mask materials and topography is the more dominating factor leading to through pitch CD errors when the scalar-Kirchhoff model is used for layout adjustment. Based on findings and classification of topography induced effects, a deviation-driver from scalar diffraction model was identified. This paper discusses a solution to compensate for topography effects while using the scalar diffraction model for reticle treatment. The area of applicability of such a scalar model, its advantages and limitations are illustrated with simulations and experiments.
The continued downscaling of the feature sizes and pitches for each new process generation increases the challenges for obtaining sufficient process control. As the dimensions approach the limits of the lithographic capabilities, new solutions for improving the printability are required. Including the design into the optimization process significantly improves the printability. The use of litho-driven designs becomes increasingly important towards the 45 nm node. The litho-driven design is applied to the active, gate, contact and metal layers. It has been shown previously, that the impact on the chip area is negligible. Simulations have indicated a significant improvement in controlling the critical dimensions of the gate layer. In this paper, we present our first results of an experimental validation of litho-driven designs printed on an immersion scanner. In our design we use a fixed pitch approach that allows to match the illumination conditions to those for the memory structures. The impact on the chip area and on the CD control will be discussed. The resulting improvement in CD control is demonstrated experimentally by comparing the experimental results of litho-driven and standard designs. A comparison with simulations will be presented.
In 2004, the successful feasibility study of immersion lithography has completely pushed back the interest in 157nm lithography. Almost the complete industry has redirected its efforts and investment to 193nm immersion lithography. IMEC has announced a new lithography affiliation program on 193nm immersion technology. The program has attracted a lot of attention and a large number of companies have joined the IMEC program in the mean time. In this paper, a status update will be given on the 193nm immersion work at IMEC. Simulation and experimental results are shared and the outlook to the future of immersion lithography will be given. Special emphasis will be put on mask related issues.
It is important to understand how a photomask will polarize incident radiation. This paper presents data collected on binary mask and various attenuated phase shifting mask materials, feature sizes, duty ratios, and illumination schemes via rigorous coupled wave analysis, extinction spectroscopy, and 193nm lithographic evaluation. Additionally, the result of polarization effects due to the photomask on imaging has been studied. It was found that in the majority of the cases, higher NA led to greater polarization effects. All mask materials predominantly pass the TM polarization state for the 0 order, whereas different materials and duty ratios affect the polarization of the first diffracted orders differently. The polarization effects contributed by mask materials being considered for use in high NA imaging systems need to be examined. The degree of polarization as a function of <i>n</i> and <i>k</i> is presented, providing an introduction to the desirable properties of future mask materials. Materials with higher refractive indices and lower extinction coefficients tend to pass more of the TM polarization state, which is undesirable. Materials with lower indices and relatively wide range of extinction coefficients pass more TE polarized radiation. The duty ratio, critical dimension, mask material, material thickness, and illumination scheme all influence mask induced polarization effects.
Through ArF immersion lithography a road towards increased optical resolution at the 193nm wavelength has been opened. According to recently proposed roadmaps, ArF immersion lithography will be used for 65nm and 45nm technology nodes. Consequently, keeping the same 4x optical demagnification factor, the dimensions on mask scale down to wavelength values when entering these nodes. Moreover CD control becomes tighter and approaches values of 2-3nm. At such conditions, topography on mask, its type and materials cannot be ignored anymore while evaluating image formation either for design analysis or OPC adjustments. The objective of this paper is to analyze the influence of mask topography on imaging. The mask topography influences polarization state and diffraction efficiencies, which are determine further image formation. Therefore these parameters and their dependence on mask type, materials and pitches are of the major concern during the analyses. We analyze the process latitude and CD variations through pitch. The complete rigorous analysis shows improved process windows with the increase of feature aspect ratio and at the same time a large through pitch CD deviation compared to the conventional Kirchhoff diffraction model.
The polarization induced by the mask is studied by using a 3D rigorous model, which solves Maxwell equations using the finite element method. The aerial image depends strongly on the change of polarization induced by the materials, thickness of the layer and pitch of the periodic masks.
The shrinking of the dimensions for each new process generation increases the challenges for lithography significantly. In order to guarantee manufacturability for future process generations, a strong interaction between lithography and design is required. A quantitative measure for the manufacturability is of key importance for driving the improvements in the design for manufacturing process. Aerial image slopes or contrasts in simulated images provide a measure for the sensitivity to process variations, but do not take the statistical process variation into account. This may result in sub-optimal choices in the design for manufacturing process.
This paper discusses the process capability analysis and provides an optimal design with corresponding imaging conditions, taking the statistical fluctuations of exposure dose and focus into account.
The mean CD value and the CD spread are calculated as a function of the amount of variation in the process variables like focus and exposure dose. Comparing these distribution parameters to the
process specifications yields the so-called process capability index as a quantitative measure for the manufacturability. Another advantage is the possibility to include the effect of mask errors on the manufacturability. Until now, however, this method had only been demonstrated for line space features. In this paper we extend the process capability analysis method for calculating the
manufacturability of arbitrary layouts. The analysis is demonstrated in an evaluation of the manufacturability of various gate layer designs, both conventional as well as litho-driven re-designs.
Assessment for introduction of immersion lithography into volume manufacturing has recently started, where one of the key focus areas includes defectivity. Particularly, the possible presence of bubbles in the immersion liquid could act as a defect source. The impact of bubbles strongly depends on their size and distance from the resist. This paper shows that a thick topcoat acts as a pellicle and suppresses the printability of the bubbles. A 1.5 μm thick topcoat has been developed especially for this purpose. A model experiment has been set to validate this approach and leads to a conclusion on the printability of defects depending on their size and distance from the resist. Both simulation and results from the model experiment are shown. In addition, a new method to detect very small bubbles will be introduced.