2001. 11.1 KIGAM Ion Beam Laboratory

MNC 2001, Japan

Ion Beam Lithography Ion Beam Lithography

Using Membrane Masks Using Membrane Masks

Y.S. Kim*, W. Hong, H.J.Woo,

H.W.Choi, G.D. Kim, J.H. LeeIon Beam Laboratory

Korea Institute of Geoscience and Mineral ResourcesGajeongdong 30, Yuseonggu, Daejeon 305-350, Korea

S. LeeDepartment of Chemistry Daejeon University

Daejeon 300-716, Korea

2001. 11.1 KIGAM Ion Beam Laboratory

MNC 2001, Japan

MotivationMotivation■ 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 doneWhat has been done■ Feasibility of the IBL using membrane masks has been studied both by simulat

ion and experiment

■ A full procedure of membrane mask fabrication has been developed

■ IBL was performed using a 2 m Si3N4 mask and a 4.5 m Si channeling with 400 - 500 keV proton beam

2001. 11.1 KIGAM Ion Beam Laboratory

MNC 2001, Japan

Advantage and Disadvantage of IBLAdvantage and Disadvantage of IBL

Advantage■ Good sensitivity for 0.1 m pattern

● X-ray : 375 mJ/cm2

● e-beam : 100 C/cm2

● IBL : 4.5 C/cm2 (720mJ)

■ Good intrinsic resolution ● 10 nm : limitation not from the wavelen

gth but from PR

Disadvantage■ In vacuum treatment■ 1:1 mask ■ lateral straggling■ non familiar method - no extensive

study

Comparison of limiting resolutions

Line Width [m]

0.01 0.1 1

cont

rast

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

multilayer resistsingle layer resist

ION

X-RAY

OPTICS

E-BEAM

Ref. : P.H. Rose, NIM B37/38, p26

2001. 11.1 KIGAM Ion Beam Laboratory

MNC 2001, Japan

Position distribution of protons entering resist surface through a 1m width slit membrane maskMembrane : 2m Si3N4

PR : 200nm PMMA

Distance from Slit Center [nm]

-2000 -1500 -1000 -500 0 500 1000 1500 2000

Num

ber

of E

vent

s [a

rbitr

ary]

0

2000

4000

6000

8000

10000

12000

350 keV

400 keV

450 keV

500 keV

14 to 86 % width

440 nm

260 nm

190 nm

160 nm

Effect of Effect of angular spreadangular spread at the membrane at the membrane on on lateral resolutionlateral resolution - - TRIM simulationTRIM simulation

Change of position distribution of protons passing through a 200nm PMMA resistafter 1m width slit membrane maskmembrane : 2m Si3N4Proton Energy : 450 keV

Distance from Slit Center [nm]

-2000 -1500 -1000 -500 0 500 1000 1500 2000

Num

ber

of E

vent

s [a

rbitr

ary]

0

1000

2000

3000

4000

5000

6000

7000

before PR

after PR

50% dose position = 505 nm14 to 86 % width = 220 nm

50 % dose position = 507 nm14 to 86 % width = 195 nm

Gaussian fit to the differentiated edge

• Meaning : Resolution depends rather on the resist contrast

2001. 11.1 KIGAM Ion Beam Laboratory

MNC 2001, Japan

Effect of Angular Spread at the Membrane to the PR patternEffect of Angular Spread at the Membrane to the PR pattern

Distance from slit center [m]

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

nu

mb

er

of

eve

nts

at

wa

fer

(=p

roto

n d

ose

)

0

5000

10000

15000

20000

25000

slit width = 1m

0.80.6

0.40.2

0.1

0.05

0.02

slit patternmembrane

wafer

400keV proton

10m

develop untilthis dose region

■ Effect of Angular Spread : Contradiction of replicated pattern for small patterns← Can be solved by the pattern size control at the mask

TRIM simulation

2001. 11.1 KIGAM Ion Beam Laboratory

MNC 2001, Japan

Angular Spread MeasurementAngular Spread Measurement

Experimental SetupAngular Distribution of protons passing through a 4.5 m [100] Si membrane

Scattering Angle [degree]

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Re

lativ

e Y

ield

0

100

200

300

400

500

600

0.41

0.42

0.46

0.45

0.38

0.46

0.40

450keV

500keV

600keV

800keV

1000keV

1000keV Au 300A

600keV Au 300A

Width(FWHM

800keV Au 300A

0.36

• Angular spread is insensitive to the incident energy

→ Other words, insensitive to the membrane thickness

GoniometerCollimator

P

SSB detector

[100] Simembrane

SSB detector

Au spoton Be

20cm

X-Y translator

2001. 11.1 KIGAM Ion Beam Laboratory

MNC 2001, Japan

Incident Proton Energy [keV]

400 500 600 700 800 900 1000 1100

FW

HM

of

angl

uar

dist

ribut

ion

(per

uni

t so

lid a

ngle

)

0.1

1

10

100

Random direction (TRIM sim.)

Channeling direction (measured)

Incident proton energy [keV]

400 500 600 700 800 900 1000 1100

Pro

ton

Ene

rgy

afte

r 4.

5m

mem

bran

e [k

eV]

0

200

400

600

800

1000

channeling direction (measured)

random direction (calculation)

Angular Spread and Residual Energy Angular Spread and Residual Energy of channeled and non channeled protonsof channeled and non channeled protons

Width of angular distribution Residual energy

● For protons passing through 4.5 m Si

2001. 11.1 KIGAM Ion Beam Laboratory

MNC 2001, Japan

Preparation of Membrane MasksPreparation of Membrane Masks

■Two kinds of masks fabricated

●non-channeling mask :

▶m low stress silicon nitride

▶Fabrication procedure very similar to the X-ray mask

●Channeling mask :

▶4.5 m Si membrane

▶Fabrication procedure as shown

[100] Si wafer

Thermal diffusion of B- 9h at 1100deg for 4.5m Si

E-beam deposition- 300A Au/ 20A Ti

Spin coating- PMMA 4000A

E-beam writing and develop

Au electroplating and lift off- Fwd-Rev pulsing method

Backside opening and etching- EDP for 10h at 105 deg

2001. 11.1 KIGAM Ion Beam Laboratory

MNC 2001, Japan

Scattering Angle [degree]

0 5 10 15 20 25

Yie

ld

0

100

200

300

400

500

600

700

1000A 2000A 3000A 4000A 5000A

crit of [100] Si :

1.05 for 450keV proton

Percent channeled protons :3.9% after 2000A

non channeledchanneled

Optimization of Pattern and membrane thicknessOptimization of Pattern and membrane thickness- - for channeling maskfor channeling mask

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

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

2001. 11.1 KIGAM Ion Beam Laboratory

MNC 2001, Japan

Proton Dose [x 1013/cm2]

0 1 2 3

Dev

elop

Spe

ed [

nm/s

]

0

200

400

600

800

standardmordmeaea10ea20ea50ea80m1i3m1i5e1i3e1i5pgpe1m2padgmeegpeegbe1m2p

at 35oC

PMMA 950k

Optimization of Resist DevelopmentOptimization 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% morpholine5% etanolamine,60% diethylenglycol - monobutylether15% distilled water

■ Choose a temperature at which the contrast becomes best

Optimum develop condition

2001. 11.1 KIGAM Ion Beam Laboratory

MNC 2001, Japan

SEM Images of Mask and Replicated PatternSEM Images of Mask and Replicated Pattern

Electroplated mask pattern

Replicated Pattern on PMMA by non-channeling mask

Mask to wafer distance : 10 m, Angular spread : 5o to 10o

Energy too large

Energy too small

Energy normal

2001. 11.1 KIGAM Ion Beam Laboratory

MNC 2001, Japan

ConclusionConclusion

■ 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.