Lecture 16 –Introduction to Optical Lithography

EECS 598-002 Winter 2006Nanophotonics and Nano-scale Fabrication

P.C.Ku

2EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku

Optical Lithography

An optical system that transfers the image from the mask to the resist layer + the process of forming an etching mask (i.e. the resist development and etc.)

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Resolution limits for imaging

Small features correspond to large (kx, ky) components.In traditional optical microscopes, the detector sees the light in the far field region.

2 2 2 2 20

2 2,max/ 2 /

x y z

x y

k k k k

k k n c k n

ω µ ε

ω π λ

= = + +

⇒ + < ⇒ =

-2 -1 1 2

2.5

5

7.5

10

12.5

15

17.5

k

2 /nπ λ2 /nπ λ− k

Resolving power=

= diffraction limit

/ nλ

( )/ 2 / 2effnλ λ≡real-spacek-space

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Finite-size lens

In a real system, the cutoff spatial frequency is often limited by the size of the lens which is quantitatively described by a numerical aperture (NA).

θ

,max,max

NA sin2sin NA

nk

kk

θπθλ

≡

⇒ = ⇒ =

Resolving power

( )/ 2NA / 2effλ λ≡

where / NAeffλ λ=

5EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku

Patterning process

resist

x

I aerial image

+

I

Dissolutionrate

Dissolutionrate

x

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Some clarifications

The minimum feature size:The fundamental limit of optical lithography is not determined by the optical system alone but rather is an overall contributions from the optics, resist, develop and etching processes.

Process window:Capability of printing small features does not always guarantee a good quality and a repeatable and controllable patterning.

Alignment:Alignment to the underlying layer is equally as important as theoptics.

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How was our prediction in the past?

1.0 µm0.7 µm0.5 µm0.35 µm0.25 µm0.18 µm0.13 µm0.10 µm ?

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ITRS prediction in 1998

ITRS 1998:

193 DUV litho cannotproduce 65 nm process.

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ITRS 1999

157 nm appears on the map.

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ITRS 2005 report

Note: 157 nmoff the chartnow.

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Major challenges (at this moment…)

Data from ENIAC.

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Evolution of optical lithography

Contact and proximity printing

1:1 projection printing

Step-and-repeat projection printing

Step-and-scan projection printing

Defects, gap control

Overlay, focus,mask cost

Reduction possible

Easier focus;better usage of lensarea

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A step-and-scan system (stepper or scanner)

Wafer

Mask

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Step-and-repeat vs step-and-scan

Step-and-repeat Step-and-scan

scan

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Evolution of optics

From Introduction to Microlithography

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An example of the optics (NA=0.6, 4X reduction)

US Patent 5969803

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Challenges in lens design

Larger lens (required by better resolution) aberrationSuitably rotating the lens in the step-and-scan system can minimize the aberration

Finite linewidth of laser source dispersionAspheric lens more expensiveTighter spec on surface quality of lensShortening the wavelength more expensive raw materials

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Resolution vs minimum linewidth

Resolution often refers to the smallest pitch of a dense line/space pattern. It is limited by the diffraction limit.

Important for DRAM/flash.

Minimum linewidth is the minimum line or space that we can resolve. It has no fundamental limit.

Important for logic chips (e.g. the gate length of a transistor)

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There’s no fundamental limit to optical lithography!

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Fundamentals of lithographic optics

DiffractionPartial coherenceDepth of focusReflection and interferencePolarization dependence

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Fraunhofer diffraction (scalar; far-field)

[ ]2 2( )

2

//

( , ) ( , ) xy

ki x yikz z

f x zf x z

e eU x y F Ui z

λλ

ξ ηλ

+

==

=

ξ

η

Mask plane

x

y

Image plane

EM field

z

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Diffraction from an aperture

[ ] /( ) sin cxf x z

axF U azλ

ξλ=

⎛ ⎞= ⎜ ⎟⎝ ⎠

a Intensity 2 2sin c axazλ

⎛ ⎞∝ ⎜ ⎟⎝ ⎠

-4 -3 -2 -1 0 1 2 3 40

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

λz/a

Before the lens

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Diffraction of a line/space (N spaces) pattern

ps

2 2

sin sin( )

sin

N px sx

I x px sx

π πλ λ

π πλ λ

⎛ ⎞ ⎛ ⎞⎜ ⎟ ⎜ ⎟

∝ ⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

-5 0 50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-5 0 50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-5 0 50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

N=5 N=10 N=100

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Basic lithographic optics configuration

maskillumination projection lens photoresist(image plane)

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Image formation

Need to have at least the 0-th and the 1st diffraction orders being collected to recover the pitch information.

+1

-1

0

0

-1

Oblique incidence can improvethe minimum pitch but resultin a less image contrast.