Applications of MoSi-based binary intensity mask for sub-40nm DRAM

In this paper, we will present applications of MoSi-based binary intensity mask for sub-40nm DRAM with hyper-NA immersion scanner which has been the main stream of DRAM lithography. Some technical issues will be reported for polarized illumination and mask materials in hyper-NA imaging. One att.PSM (Phase Shift Mask) and three types of binary intensity mask are used for this experiment; those are ArF att.PSM ( MoSi:760Å , transmittance 6% ), conventional Cr ( 1030Å ) BIM (Binary Intensity Mask), MoSi-based BIM ( MoSi:590Å , transmittance 0.1%) and multi layer ( Cr:740Å / MoSi:930Å ) BIM. Simulation and experiment with 1.35NA immersion scanner are performed to study influence of mask structure, process margin and effect of polarization. Two types of DRAM cell patterns are studied; one is a line and space pattern and the other is a contact hole pattern through mask structure. Various line and space pattern is also through 38nm to 50nm half pitch studied for this experiment. Lithography simulation is done by in-house tool based on diffused aerial image model. EM-SUITE is also used in order to study the influence of mask structure and polarization effect through rigorous EMF simulation. Transmission and polarization effects of zero and the first diffraction orders are simulated for both att.PSM and BIM. First and zero diffraction order polarization are shown to be influenced by the structure of masking film. As pattern size on mask decreases to the level of exposure wavelength, incident light will interact with mask pattern, thereby transmittance changes for mask structure. Optimum mask bias is one of the important factors for lithographic performance. In the case of att.PSM, negative bias shows higher image contrast than positive one, but in the case of binary intensity mask, positive bias shows better performance than negative one. This is caused by balance of amplitude between first diffraction order and zero diffraction order light.1 Process windows and mask error enhancement factors are measured with respect to several types of mask structure. In the case of one dimensional line and space pattern, MoSi-based BIM and conventional Cr BIM show the best performance through various pitches. But in the case of hole DRAM cell pattern, it is difficult to find out the advantage of BIM except of exposure energy difference. Finally, it was observed that MoSi-based binary intensity mask for sub- 40nm DRAM has advantage for one dimensional line and space pattern.