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Section 2: Lithography

Jaeger Chapter 2Litho Reader

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The lithographic process

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Photolithographic Process(a) Substrate covered with silicon

dioxide barrier layer(b) Positive photoresist applied to

wafer surface(c) Mask in close proximity to

surface(d) Substrate following resist

exposure and development(e) Substrate after etching of

oxide layer(f) Oxide barrier on surface after

resist removal(g) View of substrate with silicon

dioxide pattern on the surface

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Photomasks - CAD Layout

• Composite drawing of the masks for a simple integrated circuit using a four-mask process

• Drawn with computer layout system

• Complex state-of-the-art CMOS processes may use 25 masks or more

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Photo Masks

• Example of 10X reticle for the metal mask - this particular mask is ten times final size (10 μm minimum feature size - huge!)

• Used in step-and-repeat operation

• One mask for each lithography level in process

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Lithographic Process

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Printing Techniques

• Contact printing damages the mask and the wafer and limits the number of times the mask can be used

• Proximity printing eliminates damage

• Projection printing can operate in reduction mode with direct step-on-wafer

Contact printing

Proximity printing

Projection printing

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Contact Printing

wafer

hv

photoresist

Resolution R < 0.5μm

mask plate is easily damagedor accumulates defects

PhotoMaskPlate

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Proximity Printing

wafer

hv

g~20μm

exposed

Photoresist

R is proportional to ( λ g ) 1/2

~ 1μm for visible photons,much smaller for X-ray lithography

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Projection Printing

hv

lens

wafer

P.R.focal plane

~0.2 μm resolution (deep UV photons)tradeoff: optics complicated and expensive

De-Magnification: nX

10X stepper4X stepper1X stepper

Diffraction

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Aerial Images formed by Contact Printing, Proximity Printing and Projection Printing

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• Hg Arc lamps 436(G-line), 405(H-line), 365(I-line) nm • Excimer lasers: KrF (248nm) and ArF (193nm) • Laser pulsed plasma (13nm, EUV) Source Monitoring • Filters can be used to limit exposure wavelengths • Intensity uniformity has to be better than several % over the collection area • Needs spectral exposure meter for routine calibration due to aging

Photon Sources

Optical Projection Printing ModulesOptical System:

illumination and lens

Mask: transmission and diffraction

Resist: exposure, post-exposure bake and dissolution

Wafer Topography: scattering

Alignment:

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EE143 – Ali Javey

Optical Stepper

wafer

scribe line

1 2

Imagefield

field size increaseswith future ICs

field size increaseswith future ICs

Translationalmotion

Resolution in Projection Printing

Minimum separation of a star to be visible.

f = focal distanced = lens diameter

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EE143 – Ali Javey

Resolution limits in projection printing

=n.sinϴ, where n is the index of refraction

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point

Depth of Focus (DOF)

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EE143 – Ali Javey

ΔFieldOxide

Photo mask

Different photo images

Example of DOF problem

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( )

( ) .

( )

1 0 6

22 2

lNA

want small l

DOFNA

want large DOF

m m≅

= ±

λ

λ

(1) and (2) require a compromise between λ and NA !(1) and (2) require a compromise between λ and NA !

Tradeoffs in projection lithography

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Sub-resolution exposure: Phase Shifting Masks

Pattern transfer of two closely spaced lines

(a) Conventional mask technology - lines not resolved

(b) Lines can be resolved with phase-shift technology

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•A liquid with index of refraction n>1 is introduced between theimaging optics and the wafer. Advantages

1) Resolution is improved proportionately to n. For water, the index of refraction at λ = 193 nm is 1.44, improving the resolution significantly, from 90 to 64 nm.

2) Increased depth of focus at larger features, even those that are printable with dry lithography.

Immersion Lithography

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Image Quality Metric: Contrast

Contrast is also sometimes referred as the Modulation Transfer Function (MTF)

Questions:

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How does contrast change as a function of feature size?

How does contrast change for coherent vs. partially coherent light?

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* simulated aerial image of an isolated line

Image Quality metric: Slope of image

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resistresist

substrate

resist

substrate

resist

Position x

Finitecontrast

Infinitecontrast

Optical image

The need for high contrast

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Resists for Lithography

• Resists– Positive– Negative

• Exposure Sources– Light– Electron beams– Xray sensitive

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Two Resist Types• Negative Resist

– Composition:• Polymer (Molecular Weight (MW) ~65000)• Light Sensitive Additive: Promotes Crosslinking• Volatile Solvents

– Light breaks N-N in light sensitive additive => Crosslink Chains– Sensitive, hard, Swelling during Develop

• Positive Resist– Composition

• Polymer (MW~5000)• Photoactive Dissolution Inhibitor (20%)• Volatile Solvents

– Inhibitor Looses N2 => Alkali Soluble Acid– Develops by “etching” - No Swelling.

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Positive P.R. Mechanism

Photons deactivatesensitizer

polymer +photosensitizer

dissolvein developersolution

⇒

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Resist contrast Qf≡

⎛⎝⎜

⎞⎠⎟

1

100

log

Positive Resisthv

mask

P.R.

100%

E1 ETexposurephotonenergy(log scale)

resist thickness remaining

(linearscale)

exposed part is removed

Q

Qf

Q0

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Negative P.R. Mechanism

hv => cross-linking => insoluble in developer solution.

hv

E1ET

remaining

photonenergy

after development

%

mask

Q0

Qf

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Positive vs. Negative Photoresists

• Positive P.R.:higher resolutionaqueous-based solventsless sensitive

• Negative P.R.:more sensitive => higher exposure throughputrelatively tolerant of developing conditionsbetter chemical resistance => better mask materialless expensivelower resolutionorganic-based solvents

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Overlay Errors

+

+

+

+

Alignmentmarksfrom previousmaskinglevel

wafer

alignmentmask

photomaskplate

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( )sisimm TTrR αα ⋅Δ−⋅Δ⋅=

Δ ΔT Tsi change of mask and wafer tempcoefficient of thermal expansion of

mask & Si

m

m si

, .,

==α α

run-outerror

waferradius

(1) Thermal Run-in/Run-out errors

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(2) Translational Error

referrer

image

n+

Al

p

Rotational / Translational Errors

(3) Rotational Error

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Overlay implications: Contacts

n+

SiO2 SiO2

Al“ideal”

p-Si

SiO2 SiO2

Al

p-Sin+ “short”, ohmic contact

Alignment error Δ

SiO2

n+

SiO2

Al

Solution: Design n+ region larger than contact hole

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Overlay implications: Gate edgeFox“Ideal”

poly-gate

S/D implantn+ Electrical

short

“With alignment error”

Solution: Make poly gate longer to overlap the FOX

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Total Overlay Tolerance

σ σ2 2total i

i=∑

σi = std. deviation of overlay error for ith masking stepσtotal = std. deviation for total overlay error

Layout design-rule specification should be > σtotal

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Standing Waves

substrate

PositivePhotoresist

hv

substrate

After developmentPositivePhotoresist.After developmentPositivePhotoresist.

Higher Intensity

Lower Intensity

Faster Development rate

Slower Development rate

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P.R.

Intensity = minimum whenn

mdx2λ

−=

x d

m = 0, 1, 2,...

Intensity = maximum whenn

mdx4λ

−= m = 1, 3, 5,...

n = refractive index of resist

SiO2/Si substrate

Standing waves in photoresists

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Proximity Scattering

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Approaches for Reducing Substrate Effects• Use absorption dyes in photoresist• Use anti-reflection coating (ARC)• Use multi-layer resist process

1: thin planar layer for high-resolution imaging2: thin develop-stop layer, used for pattern transfer to 33: thick layer of hardened resist

(imaging layer)

(etch stop)(planarization layer)

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Electron-Beam Lithography

V312.

=λAngstroms

for V in Volts

Example: 30 kV e-beam => λ = 0.07 Angstroms

NA = 0.002 – 0.005Resolution < 1 nm

But beam current needs to be 10’s of mA for a throughput of more than 10 wafers an hour.

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Types of Ebeam Systems

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resolution factors

• beam quality ( ~1 nm)

• secondary electrons ( lateral range: few nm)

performance records

organic resist PMMA ~ 7 nm

inorganic resist, b.v. AlF3 ~ 1-2 nm

Resolution limits in e-beam lithography

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The Proximity Effect

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Richard Feynman

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EE143 – Ali Javey

EE143 – Ali Javey

Dip Pen Nanolithography

Dip-Pen Nanolithography: Transport of molecules to the surface via water meniscus.

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Dip-pen Lithography, Chad Mirkin, NWU

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EE143 – Ali Javey

Patterning of individual Xe atoms on Ni, by Eigler (IBM)

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