Lecture 16 – Introduction to Optical Lithography

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

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 2 Resolution limits for imaging

„ Small features correspond to large (kx, ky) components.

„ In traditional optical microscopes, the detector sees the light in the far fieldk region.

2 2 222 k==++ωµε0 kkkxyz

22 ⇒+<⇒kkxyω nck/2/&,max =πλ n

17.5 15 Resolving power 12.5

k-space real-space 10 = λλ/2( n)≡ eff /2 7.5 5 = diffraction limit 2.5

−2/π n λ 2/π n λ k& -2 -1 1 2 λ / n

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 3 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).

NA≡ n sinθ θ

k&,max 2π ⇒=⇒=sink&,max NA k λ

θ Resolving power Æ

λλ/2NA( )≡ eff /2

where λeff = λ /NA

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 4 Patterning process

Dissolution rate

resist x

I aerial image

x Dissolution + rate I

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

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 6 How was our prediction in the past?

1.0 µm 0.7 µm 0.5 µm 0.35 µm 0.25 µm 0.18 µm 0.13 µm 0.10 µm ?

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 7 ITRS prediction in 1998

ITRS 1998:

193 DUV litho cannot produce 65 nm process.

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 8 ITRS 1999

157 nm appears on the map.

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 9 ITRS 2005 report

Note: 157 nm off the chart now.

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 10 Major challenges (at this moment…)

Data from ENIAC. EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 11 Evolution of optical lithography

Contact and proximity printing Defects, gap control

1:1 projection printing Overlay, focus, mask cost

Step-and-repeat projection Reduction possible printing

Easier focus; Step-and-scan projection better usage of lens printing area

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 12 A step-and-scan system (stepper or scanner)

Mask

Wafer

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 13 Step-and-repeat vs step-and-scan

Step-and-repeat Step-and-scan

scan

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 14 Evolution of optics

From Introduction to Microlithography

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 15 An example of the optics (NA=0.6, 4X reduction)

US Patent 5969803

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 16 Challenges in lens design

„ Larger lens (required by better resolution) Æ aberration

„ Suitably rotating the lens in the step-and-scan system can minimize the aberration

„ Finite linewidth of laser source Æ dispersion

„ Aspheric lens Æ more expensive

„ Tighter spec on surface quality of lens

„ Shortening the wavelength Æ more expensive raw materials

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 17 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)

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 18 There’s no fundamental limit to optical lithography!

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 19 Fundamentals of lithographic optics

„ Diffraction

„ Partial coherence

„ Depth of focus

„ Reflection and interference

„ Polarization dependence

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 20 Fraunhofer diffraction (scalar; far-field)

η y ξ x z

Mask plane Image plane

k ixy()22+ eeikz 2λz Uxy(, )= FU[] (,)ξη fx =xz/λ iz f y =xz/λ

EM field

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 21 Diffraction from an aperture

⎛⎞ax FU[]()ξ = a sinc⎜⎟ fxzx = /λ ⎝⎠λz

22⎛⎞ax a Intensity ∝ a sin c ⎜⎟ ⎝⎠λz 1

0.9

0.8 0.7 Before the lens 0.6

0.5

0.4

0.3

0.2

0.1

0 -4 -3 -2 -1 0 1 2 3 4 λz/a

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 22 Diffraction of a line/space (N spaces) pattern

22 ⎛⎞⎛⎞Npxππ sx ⎜⎟⎜⎟sin sin Ix()∝ λλ s p ⎜⎟⎜⎟px sx ⎜⎟⎜⎟sin ππ ⎝⎠⎝⎠λλ

1 1 1

0.9 0.9 0.9

0.8 0.8 0.8

0.7 0.7 0.7

0.6 0.6 0.6

0.5 0.5 0.5

0.4 0.4 0.4 0.3 0.3 0.3 0.2 0.2 0.2 0.1 0.1 0.1 0 0 0 -5 0 5 -5 0 5 -5 0 5 N=5 N=10 N=100

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 23 Basic lithographic optics configuration

illumination mask projection lens photoresist (image plane)

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 24 Image formation

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

+1 0

0

-1 -1

Oblique incidence can improve the minimum pitch but result in a less image contrast.

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 25