0.1 MU-M X-RAY MASK REPLICATION

In a series of experiments, we investigate issues of absorber thickness increase (contrast amplification) and resolution capabilities of an x-ray mask copying process utilizing x-ray lithography. Two types of high resolution master masks were patterned using e-beam lithography and gold electroplating deposition and then replicated using x-ray lithography. The print quality and feature linewidths of each replica were then evaluated using a scanning electron microscope. The production of ULSI masks for x-ray lithography can be simplified if the necessary step of electron beam patterning can be performed in a thin resist layer, reducing the complexity of subsequent processing procedures and enhancing e-beam capabilities. When working with a thin resist, however, plated absorber thicknesses are small and the resulting mask contrast is limited. For x-ray lithography, Au absorber thicknesses on the order of 0.6-mu-m are required to ensure sufficient mask contrast when using synchrotron radiation (SR) sources (lambda = 0.1-1.2 nm). This implies than an aspect ratio of 3:1 is needed to print 0.2-mu-m features, and a ratio greater than 6:1 is necessary for sub-100 nm features. A promising approach to achieving greater absorber thickness in x-ray masks is based on a mask replication process utilizing x-ray lithography. The large depth of focus and wide process latitude offered by x-ray lithography (XRL) permit exposure into thicker resist layers, and consequently, greater absorber thicknesses can be produced via direct metallization of the printed image. This technique also has the advantage of reducing e-beam time requirements, since many x-ray mask replicas can be produced from a single e-beam master mask. Two types of master x-ray masks were used in our evaluation. The first was a 2-mu-m-thick SiNx membrane with sub-100-nm Au features, and the second was a 2-mu-m-thick Si membrane with Au features down to 250 nm. Using the SiNx master mask, sub-100-nm resist features were successfully printed in 600 nm of resist on base plated wafers, using x-ray lithography. Subsequent electroplating of these samples yielded sub-100 nm features with Au absorber thicknesses of 450 nm. These results demonstrate successful mask contrast amplification (from 6 dB for the master to 15 dB for the copy), and show that high resolution, high contrast masks can be fabricated using this replication process.