Contents IC technology Lithographyhome.agh.edu.pl/~marszale/downloads/W3_Lithografia.pdf · IC...

Course IC technology - file lithography.ppt 31 January 2000

Leerstoel Halfgeleider Componenten 1

IC technology

Lithography

IC technology: Lithography 2

Contents

1. Introduction

2. Overall lithography process

3. Light sources4. Optical exposure systems5. Resist materials

6. Optics basics7. Resolution improvement

8. Advanced lithography– X-Ray lithography– Electron beam lithography– SCALPEL– EUV

9. Summary




1. Introduction1.1. Lithography: basic concept

• Spin-onphotoresist

• Expose locally with UV light

• Developphotoresist

UV-light

mask

photo-resist

silicon oxide

silicon substrate


1.2. Lithography: demands

• Resolution requirements– smaller device structures

• Exposure field requirements– increasing chip size, need to

exposure at least one full chip with each exposure

• Placement accuracy– each mask layer needs to be carefully

aligned with respect to the existing pattern

• Throughput– translates into manufacturing costs

• Defect density– Translates into yield loss and higher

costs for the finished chips




1.3. Lithography: roadmap

• Miniaturisation:– More functions per chip– Increasing speed– Reduction of costs

⇒ Can be realised by lithography improvements


2. Overall lithography process

Mask making: Pattern on mask

Photoresist on substrateAlignment to previous layers

Exposure of resist - energetic radiation(UV, X-ray, E-beam, Ion, DUV, ….)

Resist development

Process step (etch, implantation, …)

Resist removal

CAD input

litho

grap

hy

Next process steps




2.1. Computer Aided Design (CAD) systems

• Patterns are designed using CAD systems, which contain:– Libraries of previous design (basic

circuits)– Software tools to wire the connections

between functional blocks– Simulation tools to predict the

performance of the new design


2.2. Mask making

(Mask made in separate mask shop)

• High precision needed

• Cost may be high!

Quartz plate

(chromium layer)




2.2. Mask making

• Mask writing is usually done with a laser or an electron beam pattern generator:

– rastering a beam in an X-Y pattern across the mask

– 4 or 5 times magnification for step and repeat systems


2.3. Lithography process, wafer exposure

• Actual lithography process can be separated into three parts:

– light sources– associated with the

photoresist




3. Light sources

Smaller feature → λ smaller!

Excimer (XUV)193 nm (ArF)

Excimer (DUV)248 nm (KrF)

Mercury lamp (UV)G-line: 436 nmI-line: 365 nm

Disadvantages of λ↓:reflections even biggerreliability of light source

157 nm(CaF2)


4. Optical exposure systems4.1. Exposure methods

Co

mp

aris

on

of a

eria

l im

ages

Mask/die arrangement:

Projection printing




4.2. Exposure tools

MESA CR

≈ R ≥ 1.5 µm

poor resolution (≈ 3 µm)mask resolution

mostly used systemR < 1.5 µm

poor-man solution


Step-and-scan systems

• Stepping accomplish the major moves from one exposure field to another

• Within each exposure field, the mask pattern is scanned across the field




5. Resist materials

• Radiation sensitive compound (organic material) resin + Photoactive compound (PAC) + solvent

• Chemical properties are changed by photochemical reactions

• Two types of resist:

exposed regions disappear exposed regions remain


5.1. Resist properties

• High contrast needed! (resolution)

γ ≈ 2 - 3

• Chemical resistance against many process steps (etchants, implants, …) → resist

• Good adhesion (primer)

1

1

ln:

−

≡

E

EContrast tγ




5.2. Lithographic transfer process


5.3. Resist application - spincoating




Spincoating track

Needed to obtain high reproducibility and high throughput - 50-100 wafers/hour

• spin (3000 - 5000 rpm)• bake• expose• bake


5.4. Typical photoresist process flow




6. Optics basics6.1. Diffraction

Diameter of central maximum = 1.22λf/d(circular aperture)


6.2. Resolution limit

Minimum feature

(diffraction limited!)

Numerical aperture

αλ

αλλ

sin

61.0

)sin2(

22.122.1

nfn

f

d

fR ===

θsinnNA =NA

kRλ

1=

75.0

6.0

)(248

1 ==

=

k

NA

KrFnmλmR µ31.0=⇒




6.3. Depth of focus

Increase in resolution leads to a reduction in focus depth

⇒ IC process must be modified when dimensions are reduced!

• More planar surface (planarisation technology)

• Less reflection (anti-reflex coating)

• Thicknesses of layers must be optimised

( ) ↓⇒↑±= DOFRNA

nDOF 22

λ


7. Resolution improvement

• Shorter wavelength

G-line → I-line → DUV → XUV

• Larger NA (≈ 0.5)

• Resist technology (high contrast, AR coatings, …)

• Mask

Optical proximity correction

Phase shift mask

• Non-optical lithography

X-Ray, electron and ion beams

λ ≤ 1-2 Å → no diffraction




• Anti reflection coatings• Higher resolution resist

7.1. Reflections of the layers

Unwanted reflections on the surface of the chip⇒ undesirable photoresist exposure


7.2. Standing waves

Highly reflective layer below the photoresist

+

Insufficient adsorption of light in the photoresist

=

Interference between the incoming and outgoing waves

⇒ Anti reflection (AR) coatings needed!




7.3. Optical Proximity Correction (OPC)

• Circular projection systems (lens)

• Rectangular features on masks

High-frequency components of the diffraction pattern are lost

⇒

=> Problem:Aerial image has rounded corners

Solution:Adjust dimensions and shapes on the mask (OPC)


7.4. Phase Shift Mask (PSM)

• Advantages: higher practical resolution and more robust process

• Disadvantages: complex mask, costs (!) and circuit layout design is specific for phase shift

Comparison of aerial image intensity of conventional (left) + Levenson-type PSM (right) mask




Phase Shift Mask

• Phase Shift Lithography: below 130 nm dimensions (memory)

• mask ≈ 100 k$


PSM: resolution improvement




(high resolution)

7.5. Resolution enhancement techniques(RET)

• Feature size < wavelength: use RET ⇒ features of approx. λ/2 can be printed


7.6. 157-nm optical lithography

• 157 nm feasible for 100 nm technology?

• Laser source technology?– CaF2 - Calcium fluoride laser – F2 - Fluorine

• Mask?– Fused silica

• Resist?

?




8. Advanced lithography• Higher resolution than optical lithography

• Sources: photons, electrons, X-rays, ions

• Printing capacities below 70 nm:

– EUV Extreme Ultraviolet Lithography (EUV)

– X-ray proximity

– Ion projection Lithography (IPL)

– SCALPEL (Scattering with angular limitation projection electron-beam lithography)


8.1. X-ray lithography

λ ≈ 1 - 2 Å




X-ray exposure system

Multiplayer mirror suitable for X-ray reflection


X-ray mask

• Thin membranes (1 µm)

(SiC, SiN, Si, ...)

• Heavy metal absorber

Au, W, ...

• Mask 1:! (very difficult)

• Mechanical stability




8.2. Electron beam lithography

Very complex equipment (mechanical, electronic, data handling)

Serial process (slow)

Very high resolution (spot size 5 nm) → << 0.1 µm

Well suited for mask fabrication!


E beam resists

PMMA↓

perspex

crosslinking

↓

Also novalak resist(Shipley SAL)




Proximity effect

Influence of nearby patterns on pattern size!

Isolated line, space, lines/spaces

• Optical proximity effect correction (OPC): uses pre-distorted amplitude patterns at the image plane to compensate for some diffraction effects

Dep

th (µ

m)

20 keV e-beam

0.4 µm resist


SCALPEL

• Scattering Limited Projection Electron Lithography (Lucent)

• +: resolution + printing nature is linear + similar resists as in DUV

• -: difficult mask, little experienceS

catt

erin

g m

ask:

1

00 n

m t

hick

me

mb

ran

e




8.3. EUV Extreme Ultra-Violet Lithography

• λ≈10 nm, soft X-ray

• all reflective optics, reflective mask

• +: looks most like stepper technology

• -: extremely complex mirror/mask technology

R~ 50 nm!DOF~ 1µm


8.4. Future

• Up to 0.1 µm optical lithography will be used!

• UV to DUV to XUV + tricks

• Phase shift mask• Illumination tricks

• Smaller dimension - ??• EUV, SCALPEL ? → 50 nm

• Continuous improvement of processes

• Stepper• Mask• Resist• Planarisation• Anti-reflection coatings

• (zie Okazaki IEDM ‘96)




9. Summary

• Lithography:

– Used for reproduction of patterns

– Improvements enable miniaturisation

Mask

• Resist

Equipment

• Optical lithography

– Decreasing wavelength to improve practical resolution

– Resolution Enhancement techniques: print sizes < wavelength, e.g. Phase Shift Mask

• EUV

• SCALPEL

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