3.155J/6.152J Lecture 10: Lithography – Part 1

Prof. Martin A. Schmidt

Massachusetts Institute of Technology

10/8/2003

Outline

The Lithographic Process Basic Process

Definitions

Fundamentals of Exposure

Exposure Systems

Resists

Advanced Lithography

Recommended reading Plummer, Chapter 5

Other: Campbell, Chapter 7,8,9

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 2

IC Process

Circuit Design Process Simulation Device Simulation

Layout BOXES

Mask Shop

Design Rules

Wafers Wafer Fab

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 3

An idea A file

Wafer Fab - Lithography

Most Common Measure of Complexity # of Masks, Minimum Feature (examples)

Approximately 50% of the Process Steps

Litho Etch Oxidation/Deposition Litho Etch Implant

Anneal Metal DepositionLitho Etch Litho Etch Sinter

Oxidation

Drives Infrastructure Cleanliness

Vibration

Temperature and Humidity

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 4

Semiconductor Roadmap

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 5

Pattern Transfer Steps

Mask

Coat with photoresist

Develop

Strip resist

Expose

Etch*

*Wet etch

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 6

Definitions

Metrics Resolution

Throughtput

Registration (Alignment)

Exposure Systems - UV Projection - Fraunhofer

Proximity - Fresnel

Contact - Fresnel

Advanced

DUV, E-Beam, X-Ray, Nano-imprint

Resists Positive/Negative

Contrast

CMTF

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 7

Lithography Systems

Proximity Contact Projection

Mask

Wafer

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 8

Contact/Proximity Printing

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 9

Contact/Proximity Printing

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 9

Applies when < g < W2/

Minimum resolvable feature = ( g)1/2

Proximity Printing Limits

Minimum resolvable feature = ( g)1/2

Gap = 20 m and Source = 436 nm

3.0 m

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 11

Projection Printing

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 12

Image from a Circular Opening

Note limit of d Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 13

Resolving Features

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 14

: When the peak of one projection lands on the first zero of the other.

Rayleigh Limit

Rayleigh Criterion

S. Wolf, Microchip Manufacturing, Lattice Press

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 15

R

n=1 (air)

The Rayleigh Criterion

R = 1.22 f/d = 1.22 f/n(2fsin ) = 0.61 /nsin

NA = nsin Range from 0.16-0.76 )

R = 0.61 /NA = k1 /NA (practical k1 = 0.6-0.8)

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 16

Modulation Transfer Function (MTF)

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 17

Inte

nsi

ty

Inte

nsi

ty

Modulation Transfer Function (MTF) Mask

MTF =

Imax - Imin

Distance Imax + IminWafer

Imax

Imin

Fall 2003 – M.A. Schmidt Distance 3.155J/6.152J – Lecture 10 – Slide 18

MTF vs Feature Size

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 19

Depth of Focus

f

d

/4 = - cos

Small : /4 = 2/2

= sin = d/2f = NA

Depth of Focus = = /2(NA)2

= k2 /(NA)2

R = 0.61 /NA = k1 /NA

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 21

Resist Contrast Dose = Intensity (W/cm-2) x time (s)

Positive Negative

Develo

ped T

hic

kness

Q0

Qf

Develo

ped T

hic

kness

Q0

Qf

Exposure Dose (log scale) Exposure Dose (log scale) mJ cm-2 mJ cm-2

= 1 / log10(Qf/Q0)

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 22

Ideal Exposure – Ideal Resist

Dose

Distance

Resi

st T

hic

kness

Distance

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 23

Dose

Qf

Real Exposure – Ideal Resist

Distance

Resi

st T

hic

kness

Increase Time

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 24

Dose Qf

Q0

Real Exposure – Real Resist

Distance

Resi

st T

hic

kness

Increase Time

Decrease Contrast

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 25

Decreasing ‘Pitch’ D

ose

Qf

Q0

Dose

Distance Distance

Resi

st T

hic

kness

Resi

st T

hic

kness

Distance

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 26

Critical Modulation Transfer Function (CMTF) D

evelo

ped T

hic

kness

Q0

Qf

Q

CMTF =

f – Q0 10 – 1 =

Qf + Q0 10 + 1

If = 3, CMTF = 0.37= 1 / log10(Qf/Q0)

If = 2, CMTF = 0.52

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 28

Effect of Coherence on MTF

CMTF

CMTF

Fall 2003 – M.A. Schmidt 3.155J/6.152J – Lecture 10 – Slide 30