X-ray phase-mask : Nanostructures

In this paper we report the results of simulations and experiments on application of phase-shifting mask to X-ray lithography (XPM). We have built an XPM with PMMA as a phase shifter; we printed patterns using the XPM and characterized the linewidth dependance on gap and dose. Small lines around 70nm were printed at 25μm gap. The resist lines have good uniformity, and aspect ratio as high as 4. The characterization experiment was performed on the Karl Suss X-ray stepper installed at the Center for X-ray Lithography. The results show that the exposure-gap latitude window is obviously increased compared with traditional X-ray mask. XLITH has been used to analyse the aerial image under the XPM and predict linewidth. Different phase shift materials, such as PMMA, Si, SiC and Al have been simulated and chacterized with respect to blur, dose and gap. The model predicts a mask contrast 6-7, and resolution of 50nm using gaps around 5μm. XPM is a path to nanostructures with more resonable process latitude. We printed sub-0.1μm dots using XPM. The XPM can also be used for gate-level lithography of ultrafast MOS devices to share its high resolution and large process latitude. It is possible to form both of a fine gate line and a contact pad at the same time with the combination of phase and transmission in a single mask. An experimental demonstration of printing unclosed lines is given in this paper. The further combination of phase-edge effects and phase-shifter size can also make it possible to form a wide range of patterns, ranging from line-space to isolated bright or dark areas with varied sizes, in a single exposure.