Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

1

Photolithography

• Minimum FeatureResolution

• Depth of Focus• Overlay Errors• Photoresist Response• E-beam and EUV lithography

Key Topics:Photo = s = (through) lightLitho = s = stoneGraphy = = writing

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

2

•Slow• several nm resolution

•High throughput

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

3

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

4

Contact Printing

wafer

hv

photoresist

Resolution R < 0.5m

mask plate is easily damagedor accumulates defects

PhotoMaskPlate

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

5

Proximity Printing

wafer

hv

g~20m

exposed

Photoresist

x

R = k ( g ) 1/2

~ 1 m for visible photons,much smaller for X-ray lithography

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

6

Projection Printing

hv

lens

wafer

P.R.focal plane

~0.2 m resolution (deep UV photons)tradeoff: optics complicated and expensive

De-Magnification: nX

10X stepper4X stepper1X stepper

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

7

Photon sources

Hg Arc lamps 436(G-line), 405(H-line), 365(I-line) nm Excimer lasers: KrF (248nm) and ArF (193nm) Laser pulsed plasma (13nm, EUV)

Source Monitoring

Filters can be used to limit exposure wavelengths Intensity uniformity has to be better than several % over the collection area Needs spectral exposure meter for routine calibration due to aging

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

8

Excimer Laser Stepper

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

9

Optical Stepper

wafer

scribe line

1 2

Imagefield

field size increaseswith future ICs

Translationalmotion

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

10

Aerial Images formed by Contact Printing, ProximityPrinting and Projection Printing

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

11

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

12

Line Patterns to illustrate principle of Projection Printing

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

13

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

14

waferplane

parallelopticalbeam

Line grating withspatial frequency 1/P

P P=L+S

...,2,1,0sin

nnP

-1

-2

+1

+2Mask lens

L S

0

sin = NA of lens

Qualitative Explanation of image degradation by lens

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

15x

x

Image on wafer After optical system

Mask Intensity

Imax

O

O

Imax

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

16

n=0

n=0 + n=1

n=0 + n=1 + n=3

n=0 + n=1 + n=3 + n=5

Effect of Fourier Components on aerial image of arectangular waveform

n=0

Source: Chapter 8 , iSheats and Smith,Microlithography

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

17

Why lNAm ?

The lens has to collect at least the n =1 diffracted beamsto show any spatially varying intensity on wafer.

Therefore printable Pminimum = /sin = /NA

For lm (L+S)/2 = Pm/2 /NA

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

18

point

best

off

Depth of Focus (DOF)

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

19

No defocus

Defocus increases

Note degradation ofimage contrastand image slope

Simulated aerial images with various degree of defocus

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

20

FieldOxide

Photo mask

Different optical images

Example of DOF problem

Step height > DOF

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

21

For Reference onlyFor Reference onlyBest focus Extreme Defocus

Large P features

Small P features

Focus versus Extreme Defocus (an illustration)

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

EE243 S2010 Lec 1122

First-Order Projection Printing Considerations

1) Minimum feature resolution lm = k1 (/NA)

2) Depth of Focus DOF = k2 / (NA)2

•NOTE: NA has contradictory effects on lm and DOF

where k1 and k2 are the technology factors

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

23

Image Quality Metric: (a) Image Contrast

Prefer high Contrast

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

24

* simulated aerial image of an isolated line

Image Quality metric: (b) Slope of Image Intensity

Prefer large Slope

Professor N. Cheung, U.C. Berkeley

Lecture 4EE143 F2010

EE243 S2010 Lec 11

Aerial Image Intensity on wafer

Photomask Pattern

Photomask Layout Pattern is 2D; Needs 2D Fourier Transform