measurement of the role of secondary electrons in euv resist exposures

cnse.albany.edu [email protected]

Measurement of the role of secondary

electrons in EUV resist exposures

June 12, 2013

International Workshop on EUV Lithography

Greg Denbeauxa, Justin Torok,a Ryan Del Re,a Henry Herbol,a

Sanjana Das,a Irina Bocharova,a Angela Paolucci,a Leonidas E. Ocola,b

Carl Ventrice Jr.,a Eric Lifshin,a Robert L. Brainarda

a. College of Nanoscale Science and Engineering, University at Albany

b. Argonne National Laboratories


Why we care about electrons for EUV exposures

EUV

hn

e-

“Universal” curve for electron

inelastic mean free path (IMFP)

Electrons generated

are near 80 eV, then

lose energy

• What energy electrons are

generated

• How far do they travel

• What energies are present at

which distances

• What energies cause PAG

decomposition

Seah and Dench, 1979

Is PAG reaction

ionization (near 10 eV)

or molecular excitation

(near 2-3 eV)?

2


MODEL

Inorganic

Materials

Photoelectron

Spectroscopy

E-Beam Depth Studies

• Development

• Mass Spec.

Electron Energy

Loss

Spectroscopy

(EELS)

PAG

• Mechanistic Analysis

• Quantum Yield

Different Material Sets/

Molecular Orbital Table

Better EUV Resist Performance:

Higher Quantum Yield, Lower Z-Parameter

Secondary electron program overview

3


How far do they go? Important for understanding

Resolution and LER

• From the central absorption event,

there will be a maximum range

(laterally) for the electrons.

• We measure the range by top down

exposures and measuring the depth to

represent the lateral electron travel

away from the EUV absorption site in

real exposures.

Bake and

Develop

Vary Dose

& Voltage

e- e- e-

Resist

Thickness Loss

(Ellipsometry)

Resist

EUV

hn

E-Beam Penetration Study:

• Expose commercial CAMP resist with 5-2000 eV

Electrons

• Bake, develop and measure penetration using

Spectroscopic Ellipsometry

4

e-

E-beam depth of penetration studies


ASML EUV ADT

EUV MET

ERIC tool (this project)

EUV ROX (outgassing ) SEMATECH EUV MBDC

Another building just

completed here

The CNSE facility

5


• Expose resist from 5-2000 eV across a range of doses.

• Bake and Develop

• Measure the thickness lost with ellipsometry (Woollam M-2000)

Mass spectrometer

E gun

Sample

Manipulator

Electron Resist Interaction Chamber (ERIC)

6


0

20

40

60

80

100

120

Thic

kne

ss (

nm

)

Expose

(vary energy and dose)

Bake and develop

Ellipsometry for thickness

measurement

Process flow for depth measurements

Thickness lost is where sufficient reactions occur – not final stopping point of electrons

7


0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80 90 100

Thic

kne

ss lo

st (

nm

)

Dose (uC/cm2)

0

10

20

30

40

50

60

0.1 1 10 100 1000

Thic

kne

ss lo

st (

nm

)

Dose (uC/cm2)

1000 eV 500 eV

100 eV

50 eV

1000 eV

500 eV

100 eV

50 eV

Electron penetration results – commercial resist

8

Higher energies penetrate deeper in resist – as expected

Thickness loss doesn’t saturate – indicating statistical distribution of electron penetration


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 1 2 3 4 5 6 7 8 9 10

Nu

mb

er

of

rele

van

t re

acti

on

s (A

U)

depth in resist (nm)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 1 2 3 4 5 6 7 8 9 10

Nu

mb

er

of

rele

van

t re

acti

on

s (A

U)

depth in resist (nm)

Illustration of process to measure relevant reactions at each depth from thickness loss

Assume there is a threshold number of reactions for clearing

Assume higher doses doesn’t physically change structure

Determine relevant reactions from penetration measurement

9


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 5 10 15 20 25 30

nu

mb

er

of

rele

van

t re

acti

on

s (A

U)

depth (nm)

500 eV

100 eV

50 eV

0

10

20

30

40

50

0.01 0.1 1 10 100 1000

Thic

kne

ss lo

st in

nm

Dose in uC/cm2

Result indicates that electron energy for reactions

with PAG occur near 5-10 eV – or the electrons

would cause reactions deeper in the resist

For 50-100 eV electrons,

typical depth for reactions

is 2-3 nm

Determine relevant reaction depth

Results from measurement of commercial resist

10


0

10

20

30

40

50

60

70

80

90

100

0.1 1 10 100 1000

Thic

kne

ss lo

st (

nm

)

Dose uC/cm2

1000 eV

500 eV

100 eV

50 eV

Similar result for PMMA

Method works for resists regardless of resist design

11


• If all charge trapped in top surface of 60 nm film with relative permittivity of 2

𝜙 =𝜎𝑑

𝜀0𝜀𝑑 , Electric potential f, dose (charge/area) s, film thickness d, permittivity of free

space e0, relative permittivity ed

• Then would build up voltage

• Until dielectric breakdown

– (near 30V for 5 MV/cm material)

• BUT… we see sample discharging

at much lower dose

0

5

10

15

20

25

30

35

40

0 0.2 0.4 0.6 0.8 1

Ele

ctri

c P

ote

nti

al (

V)

Dose (uC/cm2)

Dielectric

breakdown

Other

discharge?

What happens to the electrons?

12


For 10 and 20 keV electrons:

– Charge builds up at lowest doses then is

stable – radiation induced conductivity

– Takes hours to discharge after exposure

– Is more negative for thicker films (more

trapped electrons)

J. Vac. Sci. Technol. B 17.6., Nov/Dec 1999

Past work with higher energy electrons

13


Measure where electrons travel – not just where electrons cause reactions

Direct measurement of electron penetration

Wafer

Conductive layer

Resist

picoammeter

Kapton insulation

Measure some reflected electrons with Faraday cup

Measure transmitted electrons through resist

Expose selected energies and currents

14


Literature search for Kapton secondary electron yield

Not photoresist, but example polymer

Secondary electrons affect measurement

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 500 1000 1500 2000 2500 3000

Seco

nd

ary

ele

ctro

n y

ield

(SE

Y)

Incident electron energy (eV)

From David Joy Database

Gross et al., 1983, IEEE Trans. On Electrical Insulation

Willis and Skinner, 1973, Solid State Comm.

Yang and Hoffman, 1987, Surf. Interf. Anal.

Wide variation in literature results

Secondary electron yield > 1

will give opposite current in measurement

Surface contamination

and charging may affect

results, especially for

energy below 1000 eV

15

cnse.albany.edu [email protected] 16

Transmitted current measurements

-80

-60

-40

-20

0

0 10 20 30 40 50

Cu

rre

nt

(nA

)

Time (s)

1500eV, 200nA incident

• Typical result that we want to see

• Current transmitted through

sample when electron gun is on

• Rise and fall time are picoammeter

response time

Lower energy is not as repeatable– charging effects?

-5

-4

-3

-2

-1

0

1

0 10 20 30 40 50

Raw

Cu

rre

nt

(nA

)

Time (s)


-10

0

10

20

30

0 10 20 30 40 50Raw

Cu

rre

nt

(nA

)

Time (s)



Transmitted current measurements

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0 500 1000 1500 2000 2500 3000

Frac

tio

n t

ran

smit

ted

th

rou

gh r

esi

st (

I/I 0

)

electron energy (eV)

Electrons reach the substrate for low incident energy

much deeper than reactions occur in the resist

Lower measured transmission

at energies where secondary

electrons are most likely to

emit

Excluded 6/4 data – all results from that sample are outliers

Excluded low current < 10 nA experiments


Reflected current measurements

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0 500 1000 1500 2000 2500 3000

Frac

tio

n r

efle

cte

d t

o F

arad

ay c

up

(I/

I 0)

electron energy (eV)

Reflected signal closely matches secondary electron yield from literature


Low Energy Electron Scattering in Solids Monte Carlo Modeling Program

LESiS can start with photons or electrons and map photoelectrons and secondary

electrons as they are created and destroyed in a solid film.

Atomic Interactions Currently Part of the Model:

Elastic Scattering:

e-

= Core Electrons

= Valence Electrons

DE ≈ 0

e- e-

e- Ionization:

DE = 6-14 eV

Ionization Energy

e- e- Plasmon

Generation:

DE ≈ 3-12 eV

A plasmon is a wave of bound

valence electrons in a solid

Currently Not

being Included:

DE ≈ 3-8 eV

M M* Internal Excitation

(Similar to Photolysis)

Also not yet included:

• Molecular interactions

• Creation of Phonons

• Energy lost as heat.

LESiS modeling program

19


Same commercial resist used for penetration measurements

Secondary e-

Secondary e-

Input e-

(80 eV)

2 nm

2.5

nm

Interactions principally with the p orbitals

of Carbon and Oxygen:

Electron trajectory simulations

20


Conclusions

• EUV resist exposures are based on electron chemistry

• We have developed a flexible experimental system to

help understand these electron reactions

• We have measured the electron blur directly

• We are using the data to help optimize the simulation

software

• We plan to • Determine the actual number and energy of electrons present in the

resist due to EUV exposure

• Determine the PAG reactivity – the cross section versus electron

energy

– In order to help in the development of improved efficiency resists

21


Acknowledgments Thanks for helpful discussions

Ognian Dimov (Fujifilm)

Kevin Cummings (SEMATECH)

Thanks to CNSE for funding this work

22

measurement of the role of secondary electrons in euv resist exposures

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