challenges for lithography in tfh manufacturing for lithography in tfh manufacturing september 18,...

Challenges for Lithography in TFH Manufacturing

September 18, 2008Peter ten Berge, ASML

Outline• Introduction

• HDD Areal density roadmap• ITRS and TFH litho roadmaps• IC & TFH: many differences

• Imaging• Optical lithography strategies • Pareto for CD control budget• Further product CD control

• Overlay• Overlay trend • AlTiC & Si properties• Effect of thermal fluctuations • Colinearity

HDD Areal density roadmap

0

200

400

600

800

1000

2007 2008 2009 2010 2011 2012

year

area

l den

sity

[Gbp

si]

2007-2008 demo’s based on DTR and conventional media have

already shown >600 Gbpsicapabilities

This density compares with 1TB 2.5” HDD

(w/2-Platters)

Latest product announcements

Source: various industry publications

0

50

100

150

200

250

0 200 400 600 800 1000

areal density [Gbpsi]

track

pitc

h, b

itlen

gth

[nm

]Trackpitch reduction is the road to areal density increase

Bitlength reduction is at a hard stop due to minimum

sensor thickness

Source: various industry publications

Trackpitch

ITRS & TFH litho roadmaps seem to be converging

Source: ITRS 2007, HGST @ IDEMA conf Dec 2006

High end IC & TFH litho: many differences

IC TFH Comment

Wafer material Si AlTiC 2x heavier / m2

Wafer diameter 300mm 150-200mm risk of obsolete equipmentWafer thickness 775µm 1200-2000µm not a SEMI standard

Dies / wafer ~1,000 40,000-70,000 fewer wafers requiredField wafer layout n.a. rowbar layout absolute grid

Dielectric thickness < 0.3µm > 3µm tight OPOResist thickness < 0.2µm 0.2-1.0µm wavelength / resist choice

Feature type dense L&S iso line & trench litho extendibility Wavelength ArFi ArF / KrF NA / λ vs. dose / focus Throughput scanner limited track limited "idle" time scanner

• Nevertheless the TFH industry is (necessarily) using litho tools that have their roots in the IC industry

Optical lithography strategies

CDdense L&S = k1 * λ / NA DOFdense L&S = k2 * λ / NA2

• IC dense lines & spaces shrink follows Raleigh equation:• Reduce λ (436nm → 365nm → 248nm → 193nm → 13.5nm)

Resist formulations, light sources,...• Increase NA (Larger lenses, introduction of immersion)

New processes, polarization effects,..• Reduce k1 (RET)

OPC, PSM, off axis illumination,…

• TFH iso features not (linearly) proportional to λ / NA• Shrink driven by improved contrast / focus / dose performance• Full toolbox of optical tricks (k1) is applied

Contrast simulations suggest KrF and ArFextendibility can be achieved using reticleenhancement techniques

0

1

2

3

4

0 30 60 90 120 150nominal ISO line CD [nm]

Nor

mal

ized

Inte

nsity

Log

Slo

pe [a

.u.]

ArFKrFArFKrF

0

30

60

90

120

150

0 30 60 90 120 150nominal ISO line CD [nm]

ISO

line

CD

@ T

F [n

m] ArF

KrFArFKrF

Binary maskBinary mask

Alt PSM maskAlt PSM mask

1:20 ISO lines, NA 0.8, sigma 0.4 conv., no AF

Pareto for ISO line CDU budget

• Four main contributors to CD uniformity error are identified • These contributors are affected by several parameters

: litho system / lens related

PARETO ISO line CDU budget @TF1

2

3

4 Wafer flatness

5, ….

Focus

Reticle NCE

Dose

Various smaller contributors

Pareto for CDU – Focus - system

0

20

40

60

80

100

0 50 100 150 200

Critical Dimension (L&S)

Rel

ativ

e fo

cus

capa

bilit

y [%

]

ArF (i)KrFArF (i)KrF

• Focus capability is hardly related to wavelength or technologynode, but much more to innovations on system platform; main driver is the 200mm – 300mm transition

Note: focus cap. data may not be accurate due to differences in measurement

Pareto for CDU – Focus - lens

0

20

40

60

80

100

0 50 100 150 200


Rel

ativ

e IP

D [%

]

0

20

40

60

80

100

0 50 100 150 200


Rel

ativ

e ab

erra

tions

leve

l [%

]

• Lens-related focus components of KrF are relative good, while focal sensitivities are similar for ArF / KrF



248nm 193nm

Alt-PSM, ISO line CD = 60nm

NA/σ 0.8/0.92/0.72

Note: aberr. data may not be accurate due to differences in measurement

Pareto for CDU – Dose control

0

20

40

60

80

100

0 50 100 150 200


Rel

ativ

e do

se re

prod

ucib

ility

[%]

0

20

40

60

80

100

0 50 100 150 200


Rel

ativ

e do

se a

ccur

acy

[%]

Alt-PSM, ISO CD = 50nm

λ = 248nm, NA/σ 0.8/0.3

• Dose control improvements are implemented mostly on platform level and support KrF iso line extendibility (further than for dense L&S)

Pareto for CDU – Wafer flatness / Reticle NCE

• Wafer site flatness of < 50nm for large exposure fields is required for sub-60nm iso features*; the smaller fields used TFH manufacturing make the requirement slightly “easier”

• Mask Error Enhancement Factors > 1 can be expected in the sub-50nm iso feature region**

Source: * ITRS 2007, ** C. Mack, Field guide to optical lithography 2006

Further improvements by Imaging CD control

• Lithography is the only technique in a (TFH) process flow that can control CD uniformity on a local level; this can be used to deal with non-uniformity caused by external sources like etch, mask, track, CMP and deposition

• Full Wafer CD uniformity corrections can be performed based on:

-2%

0%

2%

-13 0 13Slit position [mm]

Inte

nsity

cha

nge

-2%0%2%4%

-4%-14 -7 0 7 14

Dos

e C

hang

e

Requested dose

Dose on Wafer

Scan position [mm]

Field-by-field Intrafield in scan direction Intrafield scan direction

Conclusion Imaging CD control

• Wavelength and NA do not play a dominant role for ultimate iso line imaging; extendibility of KrF & (dry) ArF litho tools depends on performance improvements of focus and dose control

• These performance improvements have become available in line with KrF and (dry) ArF litho tool roadmap

• Product wafer flatness / CMP capability and reticle quality will need to be improved

• Actual litho roadmap for individual TFH manufacturers also depends on resist process availability (wavelength and TFH specific etch-resistance, selectivity and thickness)

Steep overlay trend in IC is beneficial for TFH • Overlay trend is hardly related to wavelength but more to

innovations on system platform; main driver is the 200mm –300mm transition

0

20

40

60

80

100

0 20 40 60 80 100 120 140 160


Rel

ativ

e ov

erla

y [%

]

In TFH every nm of overlay improvement is welcome, with

current on-product performance approaching 10nm, and single-machine

overlay < 6nm

AlTiC vs Si substrate material properties

AlTiC (64% Al2O3 - 36%Ti - C) Si

Phase Amorphous Crystalline

Thermal expansion (ppm/C) 7.5, isotropic 2.5, anisotropic

Thermal conductivity (W/m.K) 25 150Heat capacity (J/KgK) 750 700

• Wafer temperature (drift) control is important for high end IC for SMO < 10nm

• AlTiC wafers are much more sensitive to thermal fluctuations than Si wafers, due to 3x larger CTE and 6x lower thermal conductivity

Typical temperature accommodation takes much longer for AlTiC than for Si

Thermal fluctuations lead to large AlTiC wafer magnification effects

0.00

0.20

0.40

0.60

0.80

0 30 60 90 120 150Time [s]

Waf

er E

xpan

sion

[ppm

]

AlTiCSi

Exposure time of a wafer falls in this range

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0 30 60 90 120 150Time [s]

Drif

t Rat

e [0

.1pp

m/s

]

AlTiCSi

Additional measures (compared with Si) have been required to achieve sub-10nm SMO on AlTiC

0

100

200

300

400

500

600

-5 -4 -3 -2 -1 0 1 2 3 4 5overlay [nm]

# of

sam

ples

X (mean + 3s)Y (mean + 3s)

Lot of 3 AlTiC wafers

Exposure time of a wafer falls in this range

Colinearity in rowbars – back-end requirement

Lot of 3 AlTiC wafers

• Back-end process of rowbars adds requirement to minimize short wavelength position variations across the rowbar

• Work done in high end IC on absolute grids (for matching purposes and/or dual stage tools) may help for future improvements

Measurement mark

Row bar length

dY

Least mean square fit

Measurement mark

Row bar length

dY

Least mean square fit

0

2

4

6

8

10

1 2 3 4 5 6 7 8 9

rowbar #

3 si

gma

Col

inea

rity

[nm

]

Conclusion• Continued tool overlay improvements support the steep TFH

overlay requirements roadmap

• Additional measures must be taken for TFH manufacturing on AlTiC in order to control thermal fluctuations and drifts effects

• Both for (KrF and ArF) imaging and overlay requirements litho tools are available to support the areal density roadmap

• Actual litho roadmap for individual TFH manufacturers also depends on developments in resist process, reticle quality and wafer flatness capability

Acknowledgements

Presentation contains the highly appreciated input of Oleg Voznyi, Andre Derksen and Pascale Maury of ASML system and applications

engineering departments

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