Computational Study on UV Curing Characteristics
Based on Stochastic Model
M. Koyama, M. Shirai, H. Kawata, Y. Hirai and M. Yasuda
Department of Physics and Electronics, Osaka Prefecture University,
1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japanhirai@pe.osakafu-u.ac.jp
In UV nanoimprint lithography (UV-NIL), when the pattern size becomes smaller below
several tens nanometers, curing characteristics of UV resist could be affected by statisticalfluctuation due to decrement of the total amount of molecules, which will affects the UV curingprocess. We proposed the stochastic simulation model for UV curing process and investigated thebasic polymerization characteristics [1, 2]. In this work, we study the impact of initiatorconcentration. Also, we newly introduces volume shrinkage by curing to investigate resist profile
modification.
A schematic illustration of the model is shown in Fig. 1 as follows: (a) Monomer and photo
initiators of resist is expressed as a unit cluster molecule. Molecules are placed in space randomlyand equilibrated by molecular mechanics (MM) method using Lennard-Jones potential. (b) The
reaction begins when the initiator is activated by UV exposure. The activated molecule reacts witha randomly selected unreacted monomer within a reaction radius and a chemical bond forms
between them. (c) The bonded monomer is then activated and the chain reaction occurs. (d) Whenno unreacted monomer is located within the reaction radius, the monomer diffuses and the reaction
radius is extended up to the critical radius. (e) When the activated monomers react with each other,
the chain reaction terminates. (f) Finally, the molecular positions are recalculated using the MM
method and volumetric shrinkage due to polymerization was reproduced with the equilibrium
particle distance according to the number of polymerizations.
Figure 2 shows the molecular weight distribution of a 50 nm cubic nonfunctional virtual resist
with different initiator concentrations. With increment of the initiator, the average molecular weightdecreases. In addition, the higher the concentration of the initiator, the higher the conversion rate.
These results well qualitatively reproduces the UV curing characteristics reported by experiments
in elsewhere.
Figure 3 shows the pattern profile modification due to shrinkage. The resist material is an acrylicbased virtual resist with the initiator concentration of 5%. We demonstrate the shrinkage of a thinfilm with a thickness of 20 nm (Fig. 3-a) and a line & space pattern with a residual film thicknessof 20 nm, line width of 10 nm, and the height is 10nm (Fig. 3-b). The conversion of the thin film
resist and the line & space pattern resist were 77% and 80%, respectively. The shrinkage of the thinfilm resist was about 6.2% from the change in film thickness.
As demonstrated above, the UV curing phenomena is successfully expressed using newly proposed
stochastic model[1] M. Koyama, et al., Jpn. J. Appl. Phys. 56, 06GL03 (2017)[2] M. Koyama, et al., J. Vac. Sci. Technol. B 35, 06G307 (2017).
[3] R. Suzuki, et al., J. Photopolym. Sci. Technol. 25, 2, (2012).

Unreacted
monomer
Reactedmonomer(Polymer)

(a)

(c)

(b)

ShrinkActivatedmonomeror photo
initiator
(d)

(e)

(f)

Fig. 1: Schematic illustration of the chain reaction model based on the stochastic approach.

Fig. 2: The molecular weight distribution of the polymers generated for different initiatorconcentrations.

(a) Thin-film resist.

(b) Line pattern.

Fig. 3: Pattern shapes of before and after the resist curing shrinkage. (a) Thin-film resist. (b) Linepattern.