1. 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 , Korea
S. Lee
Department of Chemistry Daejeon University
Daejeon , Korea
KIGAM Ion Beam Laboratory

2. 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 keV proton beam
KIGAM Ion Beam Laboratory

3. 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
KIGAM Ion Beam Laboratory

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

5. 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
KIGAM Ion Beam Laboratory

6. 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
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7. 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
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8. 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
KIGAM Ion Beam Laboratory

9. 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
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10. 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
KIGAM Ion Beam Laboratory

11. 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
KIGAM Ion Beam Laboratory

12. 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.
KIGAM Ion Beam Laboratory