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It-fab Italian Network for Micro and Nano Fabrication

Lithography Stefano SoresiFondazione INPHOTEC, Pisa


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substrate

resist

A technology used to create 3D patterns with feature sizes ranging from few nm up to cm.

Pattern definition is carried out by exposing to an energetic radiation the resist - a thin layer of polymeric material that is coated as a thin film onto the substrate, and used as a mask.

Combining lithography with other fabrication processes such as deposition and etching, a high-resolution topography can be produced in several materials of interest at wafer scale.

e.g. semiconductors (Si, SiO2, III-V…) - electronic integrated circuits (EICs)- photonic integrated circuits (PICs)- Micro-Electro-Mechanical-Systems (MEMS)- Micro-Opto-Electro-Mechanical-Systems (MOEMS)- Sensorse.g. 3D patterning

in the substrate by etching e.g. metallization

on the substrate by lift-off

What is lithography?


Classes of lithography

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EXPOSURE WITHOUT A MASK→ serial exposure via a focused radiation beam→ low throughput→ resolution mainly limited by radiation/matter

interaction

• e-beam lithography• Ion beam lithography• Laser lithography

collimated radiation

mask

substrate

radiation

substrate

masked

unmasked

The resist exposure has to be selective on the substrate, to define binary structures. So the lithography can be done in two modes:

EXPOSURE THROUGH A MASK→ simultaneous exposure over large areas→ high throughput→ resolution mainly limited by diffraction

• Photolithography (contact & proximity) – Mask aligners• Photolithography (projection) – Steppers / Scanners• EUV lithography• X-ray lithography

Different approach: Nanoimprint lithography - “masked”, but radiation-less: rigid master pressed onto the soft material to produce replicas


The photo-lithography concept

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(b) Proximity printing (c) Projection printing(a) Contact printing

(b) Development[ solving the less weighty molecules faster ]

(a) Exposure[ polymer chain breaking or cross-linking ]


Negative and positive resist

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mask

exposure

after development

negative resist

[ polymer chain cross-linking ]

UV light

positive resist

[ polymer chain breaking ]

exposed resist

substrate

resist


Typical photo-lithography process flow

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8) Optical inspection5) Post-exposure bake 6) Develop 7) Post-develop bake

UV Light

Mask

4) Alignment and exposure

Resist

2) Spin coat 3) Soft bake1) Vapor prime

HMDS


Substrate surface preparation

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Cleaning: remove any contaminants like grease or organic dirt on the wafers prior to resist coating (typically using acetone and then iso-propanol).

Dehydration: remove water prior to priming and coating.

Priming: (adhesion promoter) HMDS (hexa-methyl-di-silazane) is typically used before spinning resist. It reacts with the oxide surface, by replacing –OH groups on wafer surface with –CH3 groups, at the same time leaving free bonds to react with the photoresist and to improve adhesion.


Resist components

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Additives:chemicals that control specific aspects of the resist material

Solvent:gives resist its flow characteristics

Sensitizers:photosensitive component of the resist material

Resin:mix of polymers used as binder; gives resist mechanical and chemical properties


Resist spin coating

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vacuum chuck

spindleto vacuum pump

resist dispenser

Typical spinning curves:thickness depends on the resist solid content and on the spinning speed


Resist baking

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A pre-exposure bake, or soft bake, is used to remove the solvent from the resist. A typical bake is 1 minute on a 90°C hotplate or 30 minutes in a 90°C convection oven. Thick resists may benefit from a longer bake time.

A post-exposure bake, or PEB, is used to reduce standing waves inregular positive resist exposed on the steppers, or to catalyticallyenhance the photoreaction in Chemically Amplified Resists, oralso to thermally activate chemical processes such as imagereversal. It will also affect the resist profile.

Calculated No PEB PEB, 115°45 sec.

A post-development bake, or hard-bake, is sometimes used to improve a resist's wet and dry etch resistance by hardening it.It may make the resist more difficult to remove, or easier for aggressive etches. In nearly all cases, temperatures above~ 130°C will cause the resist to flow, so a DUV curing exposure is performed first to retain the profile.


Alignment and exposure

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Usually the fabrication of interesting structures requires several levels of lithography. In order toaccomplish good registration between all the levels, an alignment scheme must be worked out in theplanning stage, before the masks are made. The mask aligners are limited to a ± 1μm overlay accuracy.

UV light source

Mask

Resist

Emission spectrum of high-intensity mercury lamp

120

100

80

60

40

20

0200 300 400 500 600

Wavelength (nm)

Rela

tive

Inte

nsity

(%) g-line

436 nm

i-line365 nm

h-line405 nm

DUV*248 nm


Contact/Proximity Mask Aligner System

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Illuminator

Alignment scope (split vision) Mask

Wafer

Vacuum chuck

Mask stage (X, Y , Z , q)

Wafer stage (X, Y, Z, q)

Mercury arc lamp


Photolithography resolution limits

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(a) Contact mode + thin resist (~1um) Þ more pinholes, but best resolution(b) Proximity mode Þ resolution loss, but less pinhole defects

(a)

(b)

Near field diffraction: Fresnel approximation


e-beam lithography basics

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• Uses resist like optical lithography, but resist is sensitive to electron exposure.• Very small wavelength Þ resolution far less limited by diffraction.• At its best, electron beam is focused to a spot size ~ 5nm using electron optics.• Generate pattern by direct writing: no need of mask.• Sequential pixel-by-pixel writing: low throughput , intended for R&D and small

production , unsuitable for mass production.

)(226.1 nmVe =l

)(24.1 mVeV

hclight µl ==


Electron source

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<100> W crystal

ZrO2 reservoir

Polycrystalline W heating filament

Schottky emitters• A Schottky source is a field assisted thermionic

source.

• Schottky emitters can produce larger amounts of current compared to cold FEG systems, so more useful for e-beam lithography.

• Because they are always on (hot), organic contamination is not an issue, hence they are very stable (few % per week change in current)

• They eventually fail when the Zirconia reservoir is depleted, within 1-2 years.

Electrons can be emitted from a filament (emitter or cathode) by gaining additional energy from heat or electric field.

Three types of electron guns:• Thermionic emission gun (W, LaB6, not-sharp tip).• Field emission gun (cold, very sharp W tip, tunneling

current).• Schottky gun (field assisted thermionic emission, sharp tip).

C: cathode for emitting electronsE: extraction electrodeA1, A2: cathode lens electrode to

focus the emitted electrons


Electron optics

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An electromagnetic lens can manipulate electron trajectory to form a small electron probe.If the image rotation is ignored, the electromagnetic lens behavior can be described by the formula used for optical lens: 1/p+1/q=1/f.


Aberrations

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DOLC: disk of least confusion

ds = 0.5Csa3

a

Spherical

DOLC

dc= Cca×DE/E0 (or DV/V)

a

Chromatic

dd = 0.61l/NA = = 0.61l/sina » 0.61l/a

Diffraction

Beam shape at different planesda=Ca×aAstigmatism

a b c d


Overall beam spot diameter

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dv: virtual source diameterM: demagnification

Spherical aberration

Chromatic aberration

Diffraction

(assume no astigmatism)

• Beam spot size depends on acceleration voltage, because higher voltage leads to smaller chromatic

aberration, and shorter l thus smaller diffraction.

• A small beam divergence is good for aberrations, but not for diffraction, so a balance is needed.

• High resolution (~5nm) can be achieved at ~5kV for field emission (cold and Schottky) guns.

nmV

d

VVCd

Cd

Mdd

ddddd

d

cc

ss

vg

dcsg

23.1,61.0

21 3

2222

==

D=

=

=

+++=

lal

a

a


Beam spot diameter: a real example

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• a is determined by aperture size (~10-100µm), which should be selected wisely.• Typically beam diameter is NOT the limiting factor for high resolution, then large a is

good for high beam current and thus fast writing (assume beam blanker can follow).• But large a also reduces depth of focus (µ1/a2), leading to large beam spot size

(low resolution) if beam not well focused due to wafer non-flatness or tilt.

:a

spherical

source size limit dg

chromaticdiffraction

total beam diameter


e-beam lithography resolution limits

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The final resolution is due not only to the beam spot size, but mainly to the radiation/matter interaction

Scattering Þ spreading of the beam, resolution loss

• As electrons enter resist, they experience small angle scattering, effectively broadening the initial beam diameter.

• Forward scattering is minimized by using the thinnest possible resist and highest accelerating voltage. Effective beam diameter:

• As electrons pass through resist and enter substrate, many will undergo large angle scattering events.• These electrons may return back into the resist at a significant distance from the incident beam,

generating SE along their path and causing additional resist exposure: this is called the proximity effect

Forward scattering Back scattering (by nuclei)

Resist

Substrate


Monte-Carlo simulations on electron scattering

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Proximity effects are spread over larger areas at higher energies.

Number of backscattered electrons is not so dependent on energy, but its spatial distribution is.


Proximity effect

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• Proximity effect is negligible for isolated/sparse fine features.• It is good for areal exposure (e.g. a big square >>1µm), since pixel can be much larger than beam spot

size (right figure). For example, beam step size (pixel) of 50nm is usually enough to give uniform areal exposure, even with a beam spot size only 5nm.

• Proximity effect is worst for dense and fine patterns, such as gratings with sub-50nm pitch.

Area in-between exposed by proximity effect


Resist profile

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• Due to forward scattering and (to a less degree) back scattering, positive resist has always an undercut profile.

• Negative resist always has a tapered profile.• For patterning dense fine features, an undercut

profile often causes resist structure to collapse due to capillary force when developer is dried.

• That is, proximity effect makes patterning dense fine features difficult.

Resist

Positive resist

Negative resist

Original thickness

Developed profile

A thinner layer may be obtained after development due to exposure by proximity effect

substrate

Resist (positive) profile, not mechanically stable

Dense and fine structures


How to reduce proximity effects

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Proximity Effect Correction

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Dose modulation


Stitching error

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• At high V, deflection field size in X, Y is ~ 1mm. Stage positioning accuracy is usually ~ 1μm.

• Larger patterns Þ stitching error, i.e. gaps or overlaps between adjacent writing fields.

• Without corrective systems, sensitive devices as e.g. optical waveguides would be

discontinuous at the boundaries between writing fields.

stitching in Y stitching in X stitching in X/Y


Laser interferometer stage

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• For advanced EBL systems, laser interferometry is used to precisely measure the stage position .

• The position measurement accuracy is better than 1nm , thus the beam deflection can be compensated for writing fields misalignment.

• Using laser beam, sample height can also be monitored to maintain focusing (constant sample height).


Resist choice for typical processes

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• Which resist type to choose depends on which will give the minimum exposure area/time.

• For isolated sparse features, positive resist is suitable for liftoff process, while negative for direct

etch process.

1. Spin on positive resist

resist

2. EBL

3. Metal deposition

4. Metal Liftoff

Liftoff process using positive resist

1. Spin on negative resist

2. EBL

3. RIE substrate

Direct etching process using negative resist


Projection lithography systems – Steppers / Scanners

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• Same concept as in contact or proximity photo-lithography, but a projection system is used to reduce the mask feature size by a demagnification factor

Þ enhanced resolution

• Excimer laser light sources: l = 248 nm → 193 nm → 157 nm

• The mask is called reticle and it is moved along the horizontal plane, to expose several dies on the substrate surface (step-and-repeat mode)


EUV lithography

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• Laser Produced Plasma Source, l = 13.4 nm!

• Special reflective optics (multi-layer mirrors) for mask and projection optics

• Ultimate resolution ~nm!


X-ray lithography

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high power laser

metal tape

Shield Wall

Storage Ring

• Synchrotron radiation x-ray source (LPP much less powerful)

• l~ 10’s nm (soft x-rays) down to £1Å

• No backscatter or reflections: very fine features with verticalsidewalls.

• 1X mask technology because refractive index for all materials is

(almost) absolutely 1.0 (no lens for demagnification).

• X-ray mask difficult to fabricate with many issues: fragile, defects,

aspect ratio, bending due to heating.


Ion beam “lithography”

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• Direct milling process using a focused beam of ions (Ga)

• No resist or mask required - subtractive

• Sub-10 nm features possible

• Limited throughput

• The biggest disadvantage of FIB lithography: limited exposure depth in resist (<100nm for 100keV); thin resist makes following liftoff or etching process difficult.


Laser lithography

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• Laser light source

• Resolution comparable or better than contact lithography (0.3 um possible)

• Good throughput

• Affordable cost


Nanoimprint lithography

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• No mask, but imprint mold

• Sub-10 nm resolution demonstrated

• High throughput

• Affordable cost


Thank you for your attention

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[email protected]

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