Section 2: Lithography Jaeger Chapter 2 EE143 Ali Javey

EE143 – Ali Javey Slide 5-1

Section 2: Lithography

Jaeger Chapter 2


The lithographic process


Photolithographic Process(a) Substrate covered with silicon dioxide

barrier layer(b) Positive photoresist applied to wafer surface(c) Mask in close proximity to surface(d) Substrate following resist exposure and

development(e) Substrate after etching of oxide layer(f) Oxide barrier on surface after resist removal(g) View of substrate with silicon dioxide

pattern on the surface


Photomasks - CAD Layout

• Composite drawing of the masks for a simple integrated circuit using a four-mask process

• Drawn with computer layout system

• Complex state-of-the-art CMOS processes may use 25 masks or more


Photo Masks

• Example of 10X reticle for the metal mask - this particular mask is ten times final size (10 m minimum feature size - huge!)

• Used in step-and-repeat operation

• One mask for each lithography level in process


Lithographic Process


Printing Techniques

• Contact printing damages the mask and the wafer and limits the number of times the mask can be used

• Proximity printing eliminates damage

• Projection printing can operate in reduction mode with direct step-on-wafer, eliminating the need for the reduction step presented earlier

Contact printing

Proximity printing

Projection printing


Contact Printing

wafer

hv

photoresist

Resolution R < 0.5m

mask plate is easily damagedor accumulates defects

PhotoMaskPlate


Proximity Printing

wafer

hv

g~20m

exposed

Photoresist

R is proportional to ( g ) 1/2

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


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


Aerial Images formed by Contact Printing, Proximity Printing and Projection Printing


Optical Stepper

wafer

scribe line

1 2

Imagefield

field size increaseswith future ICs

field size increaseswith future ICs

Translationalmotion


Light is in pulses of 20 ns duration at a repetition rate of a few kHz.

About 50 pulses are used.

Excimer Laser Stepper


Photon Sources


Resolution limits in projection printing


Resolution limits: Bragg condition


Image Degradation by lens

waferplane

parallelopticalbeam

grating withspatial frequency 1/P

P P=2L

...,2,1,0sin

nnP

-1

-2

+1

+2Mask lens

L L

0

sin = NA of lens


The /NA limit

x

x

Image on wafer optical system

Mask Intensity

Imax

O

The lens has to collect at least the n=1 diffracted beamsto show any spatially varying intensity on wafer.Therefore Pm ( = 2 lm) / NA


n=0

n=0 + n=1

n=0 + n=1 + n=3

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

Resolution and the need for higher order terms


point

best

off

Depth of Focus (DOF)


FieldOxide

Photo mask

Different photo images

Example of DOF problem


( ) .

( )

1 0 6

22 2

lNA

want small l

DOFNA

want large DOF

m m

lm

(1) and (2) require a compromise between and NA !

Tradeoffs in projection lithography


Sub-resolution exposure: Phase Shifting Masks

Pattern transfer of two closely spaced lines

(a) Conventional mask technology - lines not resolved

(b) Lines can be resolved with phase-shift technology


•A liquid with index of refraction n>1 is introduced between theimaging optics and the wafer. Advantages

1) Resolution is improved proportionately to n. For water, the index of refraction at = 193 nm is 1.44, improving the resolution significantly, from 90 to 64 nm.

2) Increased depth of focus at larger features, even those that are printable with dry lithography.

Immersion Lithography


Image Quality Metric: Contrast


* simulated aerial image of an isolated line

Image Quality metric: Slope of image


resistresist

substrate

resist

substrate

resist

Position x

Finitecontrast

Infinitecontrast

Optical image

The need for high contrast


Resists for Lithography

• Resists– Positive– Negative

• Exposure Sources– Light– Electron beams– Xray sensitive


Two Resist Types• Negative Resist

– Polymer (Molecular Weight (MW) ~65000)– Light Sensitive Additive Promotes Crosslinking– Volatile Solvents– Light breaks N-N => Crosslink Chains– Sensitive, hard, Swelling during Develop

• Positive Resist– Polymer (MW~5000)– Photoactive Inhibitor (20%)– Volatile Solvents– Inhibitor Looses N2 => Alkali Soluble Acid– Develops by “etching” - No Swelling.


Positive P.R. Mechanism

Photons deactivatesensitizer

polymer +photosensitizer

less cross-linking

dissolvein developersolution


=E resist sensitivity

Resist contrast

T

log

Positive Resisthv

mask

P.R.

100%

E1 ETexposurephotonenergy(log scale)

resist thickness remaining

(linearscale)

exposed part is removed


Negative P.R. Mechanism

1

1log EET

hv => cross-linking => insoluble in developer solution.

hv

E1ET

remaining

photonenergy

after development

%

mask


Positive vs. Negative Photoresists

• Positive P.R.: higher resolution aqueous-based solvents less sensitive

• Negative P.R.:more sensitive => higher exposure throughput relatively tolerant of developing conditions better chemical resistance => better mask material less expensive lower resolution organic-based solvents


Overlay Errors

+

+

+

+

Alignmentmarksfrom previousmaskinglevel

wafer

alignmentmask

photomaskplate


sisimm TTrR

T Tsi change of mask and wafer tempcoefficient of thermal expansion of

mask & Si

m

m si

, .,

run-out error

waferradius

Thermal Run-in/Run-out errors


(2) Translational Error

referrer

image

n+

Al

p

Rotational / Translational Errors

(3) Rotational Error


Overlay implications: Contacts

n+

SiO2 SiO2

Al“ideal”

p-Si

SiO2 SiO2

Al

p-Sin+ “short”, ohmic contact

Alignment error

SiO2

n+

SiO2

Al

Solution: Design n+ region larger than contact hole


Overlay implications: Gate edgeFox“Ideal”

poly-gate

S/D implantn+ Electrical

short

“With alignment error”

Solution: Make poly gate longer to overlap the FOX


Total Overlay Tolerance

2 2total i

i

i = std. deviation of overlay error for ith masking step

total = std. deviation for total overlay error

Layout design-rule specification should be > total


Standing Waves

substrate

PositivePhotoresist

hv

substrate

After developmentPositivePhotoresist.After developmentPositivePhotoresist.

Higher Intensity

Lower Intensity

Faster Development rate

Slower Development rate


P.R.

Intensity = minimum whenn

mdx2

x d

m = 0, 1, 2,...

Intensity = maximum whenn

mdx4

m = 1, 3, 5,...

n = refractive index of resist

SiO2/Si substrate

Standing waves in photoresists


Proximity Scattering


Approaches for Reducing Substrate Effects• Use absorption dyes in photoresist• Use anti-reflection coating (ARC)• Use multi-layer resist process

1: thin planar layer for high-resolution imaging2: thin develop-stop layer, used for pattern transfer to 33: thick layer of hardened resist

(imaging layer)

(etch stop)(planarization layer)


Electron-Beam Lithography

V312.

Angstroms

for V in Volts

Example: 30 kV e-beam => = 0.07 Angstroms

NA = 0.002 – 0.005Resolution < 1 nm

But beam current needs to be 10’s of mA for a throughput of more than 10 wafers an hour.


Types of Ebeam Systems


Resolution limits in e-beam lithography


The Proximity Effect

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Section 2: Lithography Jaeger Chapter 2 EE143 Ali Javey