5. Lithographypioneer.netserv.chula.ac.th/~ksongpho/584/5.pdf · Intro to Nanoelectronics 1 5. Lithography 1. photolithography • intro: overall, clean room 2. principle 3. tools

S Kanjanachuchai

2102-584 Intro to Nanoelectronics

1

5. Lithography

1. photolithography• intro: overall, clean room

2. principle3. tools4. pattern transfer5. resolution6. next-gen

References:Semiconductor Devices: Physics and Technology. 2nd Ed. SM Sze. Ch 12 Fundamental of Semiconductor Fabrication. GS May & SM Sze. Ch 4

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Photolithography process

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Clean room:

lithography must be carried out in a clean-room environment [A] because the presence of dust particles [B] will lower yields.

[A] [B]

1. Pinhole2. Current constriction3. S/C or O/C

yield = # good chips / # total chips

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Figure 12.2.Particle-size distribution curve for

English (---) and metric (—) classes of clean rooms.4

ENGLISH SystemClass 100 = 100 particles/ft3

(particle size >= 0.5 µm)

or 100×103 particles / m3

(particle size >= 0.1 µm)

"clean" quantified:

cleanliness – cost trade-off

1 ft = 0.3048 m1 ft3 = 0.0283 m3

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Clean room & HEPA filter

HEPA = high efficiency particulate air.

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Lithography

1. photolithography2. principle

• shadow, projection3. tools4. pattern transfer5. resolution6. next-gen

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Shadow

λ

g

λ (µm) g (µm) CD (µm)

0.4 50 4.5

0.25 15 2

Contact: dust particles can be transferred between mask and wafer low yieldProximity: dust not transferred high yield (poor resolution due to diffraction) yield-resolution trade-off gCD λ≅

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Image partitioning techniques for projection printing: (a) annual-field wafer scan, (b) 1:1 step-and-repeat,

(c) M:1 reduction step-and-repeat, and (d) M:1 reduction step-and-scan.

Projection

Stepper(step-and-repeat system)

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Lithography

1. photolithography2. principle3. tools

• light source• mask• photoresist

4. pattern transfer5. resolution6. next-gen

http://en.wikipedia.org/wiki/Electromagnetic_spectrum�

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Pre-1990: high-pressure mercury-arc lamp

source λ(nm) resolution (nm)Hg 365 300KrF 248 180ArF 193 100F2 157 70

Mercury (Hg) sourceline λ(nm)G 436H 405I 365

Hg

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CMOS scaling necessitates the development of low-λ light source

Light source roadmap

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Lithography




ITRS2009. Table LITH5A

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Mask

How to make masks?Mask blank electron-beam lithography IC Mask (reticle)[Cr / SiO2] [CAD]

info, alignment marks, test structures, etc.

alignment marks

test structures

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Yield for a 10-mask lithographic process with various defect densities per level.

In order to improve yield, masks are cleaned / inspected regularly

Yield

the yield of a lithographic step depends critically on mask cleanliness, or how many fatal defects are presence:

see fig. [B] slide #3

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Lithography




source: ITRS2011, Table LITH3A

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Suppliers: AZ, Shipley, Sumitomo

There are two types of resists: positive and negative. The final pattern on wafer is the same as (+) or opposite to (–) those on mask.

It is called photo-resist because:photo – it responds to photonsresist – it resists etching / dopant atoms, protecting the layer underneath

Photoresist

photo

resist

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Figure 12.9. Exposure-response curve and cross section of the resist image after development.1 (a)

Positive photoresist; (b) negative photoresist.

The “quality” of the resist and the exposure / development process is measured by the steepness of the resist wall, or the contrast ratio:

source: ITRS2011, Table LITH3B

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Lithography

1. photolithography2. principle3. tools4. pattern transfer5. resolution6. next-gen

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Pattern transfertransfers the pattern which appears on the photoresist onto the substrate (Si). The transfer process can be subtractive [A], or additive [B] in nature.

[A] Subtractive: etching(for oxide removal, interconnect)

[B] Additive: metallization(S/D metal, via filling)

Some process (such as doping through photoresist) is neither subtractive nor additive.

Lift-Off process

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20source: ITRS2011, Fig. LITH1

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Lithography

1. photolithography2. principle3. tools4. pattern transfer5. resolution

• limit• enhancement (PSM, OPC, Immersion)

6. next-gen

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( )22

1

sin

NAkDOF

nNANA

klm

λθ

λ

=

=

=

NA: numerical apertureDOF: depth of focus

Q) How to improve resolution (lm)?A) ↓λ, ↑NA

Resolution of photolithographyminimum printed line / feature size / resolution(lm)

trigonometry:

Requirements:lm,↓, DOF↑

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Diffraction strong when L ~< λ

L

λ

Diffractiondiffraction is the limiting factor in printing two closely-spaced lines.

mask

wafer

λ>L λ≤L

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Lithography

1. photolithography2. principle3. tools4. pattern transfer5. resolution

• limit• enhancement (PSM, OPC, Immersion)

6. next-gen

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25source: ITRS2009, Table LITH1

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A schematic of an optical lithography system showing the methods that can improve the technique's performance. Key: λ is the wavelength of the illumination source; OAI is off-axis illumination; IIL is imaging interferometric illumination; OPC is optical proximity correction; and PSM is phase-shift masks.

Link

Resolution Enhancement Techniques

http://nanotechweb.org/articles/feature/3/6/1/1�

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Figure 12.12. The principle of phase-shift technology. (a) Conventional technology; (b) phase-shift technology.9

diffraction

intensity

d = λ/2(n-1)

Phase-Shift Mask (PSM)

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Optical Proximity Correction (OPC)a purely mathematical process

The automated algorithmic link between DesignGauge, the application system for CD scanning electron microscopy (CD-SEM), and Proteus OPC for pre-processing OPC model building data allows Proteus customers to seamlessly obtain a large sampling of metrology data to account for process variations across the entire process window.

http://www.hitachi-hitec.com/global/index.html�

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Immersion

θ

λ

sin

1

nNANA

klm

=

=

use n > 1

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ce: IT

RS20

11, T

able

LITH2

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High-Index Lenses Push Immersion Beyond 32 nmAaron Hand -- Semiconductor International, 4/1/2006

water's refractive index (n) = 1.44high-index fluids n~1.65photoresist n = 1.7lens (n=1.56) becomes the limiter: calcium fluoride (CaF2) higher-index lens materials: lutetium aluminum garnet (LuAG), n = 2.1

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Lithography

1. photolithography2. principle3. tools4. pattern transfer5. resolution6. next-gen

• EUV, XRL, EBL, IBL, NIL

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sour

ce: IT

RS20

11, F

ig. L

ITH3

Aso

urce

: ITRS

2011

, Fig.

LIT

H3B

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EUV: extreme-ultra violet lithography

In an laser produced plasma (LPP)-based system, EUV light is produced by bombarding a sliver of tin with a high-power laser. The light is then gathered by specially engineered EUV mirrors, which then focus an EUV beam in the EUV scanner to produce microchip patterns.

ITRS2009: Table LITH5C

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XRL: X-ray lithography

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Schematic of an electron-beam machine.

Direct-write (serial)

Figure 12.16. Schematic of positive and negative resists

used in electron-beam lithography.

EBL: electron-beam lithography

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RSiNx100-150nmsmall scattering angle

LCr/W30-60nmlarge scattering angle

RL

100 keV

Scattering with angular limitation projection electron beam lithography system

EBL: parallel

Basic SCALPEL principle of operation showing contrast generation by differentiating more- or less-scattered electrons.

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IBL: Trajectories of 60 keV H* ions traveling through PMMA into Au, Si, and PMMA.18

EBL: Proximity effect: (a) Simulated trajectories of 100

electrons in PMMA for a 20-keV electron beam.15 (b) Dose distribution for forward

scattering and backscattering at the resist-substrate interface.

IBL: ion-beam lithography

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NIL: Nano-imprint lithography

ITRS2009: Table LITH5D

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http://www.molecularimprints.com/index.html�

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Conclusions• photolithography

– process, principle, tools– resolution limits– resolution-enhancement techniques

• PSM, OPC, Immersion, DP,…• future lithographic systems

– EUV, XRL, ML2 (EBL, IBL), NIL, ...

from ITRS2009:

... To continue as the dominant technique for leading-edge critical layer lithography, resolution enhancement techniques (RETs) such as off-axis illumination (OAI), phase shifting masks (PSMs), and optical proximity corrections (OPCs) are being used with imaging systems at the 193 nm wavelength. In addition to RETs, lenses with increasing numerical apertures and decreasing aberrations will be required to extend the life of optical lithography. Liquid immersion imaging with a fluid between the final lens element and the wafer is also being used to extend optical lithography.

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5. Lithographypioneer.netserv.chula.ac.th/~ksongpho/584/5.pdf · Intro to Nanoelectronics 1 5. Lithography 1. photolithography • intro: overall, clean room 2. principle 3. tools