Modeling the interaction of EUV radiation with photoresist materials from first principles

K. D. Closser1‡ and D. Prendergast1

M. Ahmed3, P. D. Ashby1, S. Bhattarai2, O. Kostko3, Y. Liu1, D. F. Ogletree1, D. L . Olynick1, D. Slaughter3, B. Xu3, P. Naulleau2

1The Molecular Foundry, 2Center for X-Ray Optics, 3Chemical Sciences Division; Lawrence Berkeley National Laboratory‡KDClosser@lbl.gov

1

Background

Photolithography is widely used in industry for transferring patterns to silicon

chips

Smaller features are desirable due to lower energy requirements and faster

response times

As the feature size is proportional to the wavelength, shorter wavelengths are

required to reach smaller features while avoiding the extra cost associated with

multiple patterning and extra processing

• Current industrial standard (UV): 193 nm (6.4 eV)

• Next generation sources (EUV): 13.5 nm (92 eV)

High energy EUV radiation interacts with both valence and semi-core electrons

Excitation of these tightly bound electrons may result in fundamentally different

chemistry and potentially result in selective bond breaking

Challenges associated with EUV

• Low optical cross-section for

standard organic photoresists

• # photons/energy unit much

smaller requiring greater

efficiency per photon

• Secondary electrons can

damage material over a large

radius

Need materials with better

controlled chemistry, and larger

cross-sections to improve

efficiency and make new

technology viable for industry

Fig: Illustration of photolithography

with a positive tone resist.

Fig: Absorption cross-section at 92 eV, with halogens and atoms in organic resists highlighted

Project Goals

1. Increase fundamental understanding of EUV interaction with matter using calculations from first principles

• Compute properties of model resist materials such as photon absorption, electron energy loss and Auger emission

• Use molecular dynamics to investigate potential chemistry

• Combine results to create probabilistic model

2. Propose and develop new photoresist materials using insights gained

Acknowledgements

This research was supported by the Laboratory Directed Research and Development Program of Lawrence Berkeley National

Laboratory (LBNL) under U.S. Department of Energy Contract No. DE-AC02-05CH11231

Optical Absorption (Step 1)

Fig: Optical absorption of CH3X (X= H, OH, F, Cl, Br, I)

Fig: Valence and semi-core orbital

energies for CH3X

Compute vertical excitation energies at ground state

geometries (Born-Oppenheimer approximation)

Low energy valence and deep core excitations readily

computed with standard time-dependent density functional

theory (TDDFT)

Not practical when many states are required and valence

electrons can not be neglected; the Lanczos method is used

and does not require explicit computation of excited states

Increased absorption at EUV energies for F and OH due to

presence of deeper valence levels causing slow decay of

valence peak

For I the increased absorption is

primarily due to 4d electron

excitations (Br has similar behavior

but peaks near 200 eV)

At 92 eV absorption is primarily

atomic, but intensity for I is

underestimated

Difference between gas and

normalized condensed phase

absorption is minimal

Fig: Comparison of TDDFT results with

experiment (Chem. Phys. 232, 1998, 211)

Fig: Optical absorption of condensed phase methyl phenols

for two local minima A, B

Fig: Optical absorption of gas phase methyl phenols

XMePh (X= H, F, Cl, Br, I)

Approach

Fig: Cartoon representation of dominant events occurring in EUV

absorption and subsequent electronic and nuclear relaxation.

Consider dominant processes in initial absorption,

electronic relaxation and chemical changes

independently

Use density functional theory (DFT) to compute

electronic structure, including time-dependent DFT for

EUV absorption and electron energy loss spectra

Potential dissociation channels determined through

initial forces and ab initio molecular dynamics

Begin with organic resist model systems:

• Methane, methanol and halogenated methanes for

validation of the methodology (compare to existing

experiment)

• Use halogenated methyl phenols as models for

polymeric organic resists

• Investigate gas phase first and then shift to models

for a condensed phase system

Fig: Representation of timescales of processes

relevant to EUV excitation and decay.

Fig: Example of random block copolymer organic photoresist and monomers used in synthesis (J.

Photopolym. Sci. Tech, 7, 1994, 433) and substituted phenol model (X = H, F, Cl, Br, I)Fig: Condensed phase model with

relaxed geometry

Electronic Scattering (Step 2b)

Fig: Electron energy loss (EEL) for methyl phenols

Compute structure factor for information about electron decay

Most effective energy loss for methyl phenols ~10-35 eV higher and

lower energy electrons more likely to diffuse further

Selecting for electrons most likely to lose energy quickly may provide

a method to limit blur

Strongly dependent on momentum transfer, interpretation for gas

phase is unclear

Future Work

Secondary Electron Generation (Step 2a) High Throughput Screening Monte Carlo Modeling

Auger decay produces dications emits secondary

electrons at specific kinetic energies

Fig: Representation

of screening

process for

new resist

materialsFig: Various Auger processes showing formation of

dications (b), (c), (d) and trications (e), (f), (g)

Determine methodology

to effectively screen for

novel materials

Chemical Rearrangement (Step 3)

Fig: Initial forces for CH3I after ionization from semi-core or valence orbitals

Use initial forces and emptied orbitals to screen for

states that may undergo rearrangement

Use ab initio molecular dynamics to determine potential

products

Fig: Ionization from bonding valence orbital (15b) leads to C-I dissociation

timeFig: Initial forces for IMePh from formation of I (4d)

(a) cation and (b) valence dication

(a) (b)

Probabilistic evaluation of

electron decay pathways

Determination of important

charged species for further study

Fig: Possible pathways for hole

formation and decay