Ion Beam Lithography Using Membrane Masks

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 305-350, Korea
• S. Lee
• Department of Chemistry Daejeon University
• Daejeon 300-716, Korea

2
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

3
Advantage and Disadvantage of IBL
Advantage 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 11 mask
lateral straggling non familiar method - no
extensive study
Ref. P.H. Rose, NIM B37/38, p26
4
Effect of angular spread at the membrane on
lateral resolution - TRIM simulation

• Meaning Resolution depends rather on the
  resist contrast

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

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

7
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
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

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

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
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
12
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.