Ion Beam Lithography Using Membrane Masks Y.S. Kim*, W. Hong, H.J.Woo, H.W.Choi, G.D. Kim, J.H. Lee Ion Beam Laboratory Korea Institute of Geoscience and Mineral Resources Gajeongdong 30, Yuseonggu, Daejeon 305-350, Korea S. Lee Department of Chemistry Daejeon University Daejeon 300-716, Korea 2001. 11.1 KIGAM Ion Beam Laboratory

KIGAM Ion Beam Laboratory Motivation Ion Beam Lithography (IBL) using membrane masks has been forgotten for more than 10 years. The reason seems to be that the angular spread of the incident ion beam in the membrane is difficult to overcome even when channeling masks are used. Membrane mask has, however, many advantages such as rigidity, easy fabrication, durability, etc and deserves to be studied further. The angular beam spread of channeling masks (about 0.5o) is enough for obtaining sub 100nm pattern as will be shown What has been done Feasibility of the IBL using membrane masks has been studied both by simulation and experiment A full procedure of membrane mask fabrication has been developed IBL was performed using a 2 mm Si3N4 mask and a 4.5 mm Si channeling with 400 - 500 keV proton beam 2001. 11.1 KIGAM Ion Beam Laboratory

Advantage and Disadvantage of IBL Good sensitivity for 0.1 mm pattern X-ray : 375 mJ/cm2 e-beam : 100 mC/cm2 IBL : 4.5 mC/cm2 (720mJ) Good intrinsic resolution 10 nm : limitation not from the wavelength but from PR Disadvantage In vacuum treatment 1:1 mask lateral straggling non familiar method - no extensive study Ref. : P.H. Rose, NIM B37/38, p26 2001. 11.1 KIGAM Ion Beam Laboratory

KIGAM Ion Beam Laboratory Effect of angular spread at the membrane on lateral resolution - TRIM simulation Meaning : Resolution depends rather on the resist contrast 2001. 11.1 KIGAM Ion Beam Laboratory

Effect of Angular Spread at the Membrane to the PR pattern TRIM simulation Effect of Angular Spread : Contradiction of replicated pattern for small patterns Can be solved by the pattern size control at the mask 2001. 11.1 KIGAM Ion Beam Laboratory

Angular Spread Measurement Angular Distribution of protons passing through a 4.5 mm [100] Si membrane Experimental Setup Angular spread is insensitive to the incident energy Other words, insensitive to the membrane thickness 2001. 11.1 KIGAM Ion Beam Laboratory

Width of angular distribution Angular Spread and Residual Energy of channeled and non channeled protons For protons passing through 4.5 mm Si Width of angular distribution Residual energy 2001. 11.1 KIGAM Ion Beam Laboratory

Preparation of Membrane Masks Two kinds of masks fabricated non-channeling mask : 2mm low stress silicon nitride Fabrication procedure very similar to the X-ray mask Channeling mask : 4.5 mm Si membrane Fabrication procedure as shown 2001. 11.1 KIGAM Ion Beam Laboratory

Optimization of Pattern and membrane thickness - for channeling mask Pattern thickness For 450 keV protons, 200nm thick pattern is enough for scattering 96% of protons incident on the pattern easy fabrication sub 100nm patterns Membrane thickness As thick as possible provided the residual energy is enough for penetrating through the object PR (about 100keV) minimization of pattern distortion during irradiation 2001. 11.1 KIGAM Ion Beam Laboratory

Optimization of Resist Development Choice of Developer Choose a developer which shows the best contrast Contrast = slop in the dose vs. develop speed curve Best so far : 20% morpholine 5% etanolamine, 60% diethylenglycol - monobutylether 15% distilled water Choose a temperature at which the contrast becomes best Optimum develop condition 2001. 11.1 KIGAM Ion Beam Laboratory

SEM Images of Mask and Replicated Pattern Energy too large Electroplated mask pattern Energy normal Energy too small Replicated Pattern on PMMA by non-channeling mask Mask to wafer distance : 10 mm, Angular spread : 5o to 10o 2001. 11.1 KIGAM Ion Beam Laboratory

KIGAM Ion Beam Laboratory Conclusion The IBL using channeling mask was studied already about 20 years ago, but was forgotten for many years afterwards. We want to emphasize, however, the method deserves to get attention, mainly because the problem with angular spread cannot be an fatal restriction. Simulation and some preliminary experiment on the angular spread shows the promising characteristics of the method. Provided a good channeling membrane mask is fabricated, sub 0.1 um patterning can be done rather simply. 2001. 11.1 KIGAM Ion Beam Laboratory