LITHOGRAPHY

• Lithography is arguably the single most important technology in IC manufacturing.• The SIA ITRS is driven by the desire to continue scaling device feature sizes.

• 0.7X in linear dimension every 3 years. Control of CD ~10% smallest size• Placement accuracy ≈ 1/3 of feature size.• ≈ 35% of wafer manufacturing costs for lithography.• Note the ??? - single biggest uncertainty about the future of the roadmap.

Year of Production 1998 2000 2002 2004 2007 2010 2013 2016 2018

Technology Node (half pitch) 250

nm

180 nm 130 nm 90 nm 65 nm 45 nm 32 nm 22 nm 18 nm

MPU Printed Gate Length 100 nm 70 nm 53 nm 35 nm 25 nm 18 nm 13 nm 10 nm

DRAM Bits/Chip (Sampling) 256M 512M 1G 4G 16G 32G 64G 128G 128G

MPU Transistors/Chip (x106) 550 1100 2200 4400 8800 14,000

Gate CD Control 3 (nm) 3.3 2.2 1.6 1.16 0.8 0.6

Overlay (nm) 32 23 18 12.8 8.8 7.2

Field Size (mm) 22x32 22x32 22x32 22x32 22x32 22x32 22x32 22x32 22x32

Exposure Technology 248

nm

248 nm 248 nm

+ RET

193nm

+ RET

193nm +

RET

193nm

+ RET

+ H2O

193nm

+ RET

+ H2O

157nm??

??? ???

Data Volume/Mask level (GB) 216 729 1644 3700 8326 12490

Lithography - Chapter 5

SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin

© 2000 by Prentice HallUpper Saddle River NJ

Reduction of minimum size x0.7

X-ray, e-beam, extreme UV, ion beam

Historical Development and Basic Concepts

• Patterning process consists of mask design, mask fabrication and wafer printing.

CAD System• Layout• Simulation• Design Rule Checking

Electron Gun

Focus

Deflection

Mask

Mask Making Wafer Exposure

LightSource

Mask

Wafer

Condenser Lens

Reduction Lens

AerialImage

(Surface)

N Well P Well

P+ N+ N+P+

P

LatentImage

in Photoresist

TiN LocalInterconnect Level

(See Chapter 2)

• It is convenient to divide the wafer printing process into three parts A: Light source, B. Wafer exposure system, C. Resist.

• Aerial image is the pattern of optical radiation striking the top of the resist. • Latent image is the 3D replica produced by chemical processes in the resist.

A. Light Sources

• Decreasing feature sizes require the use of shorter

• Traditionally Hg vapor lamps have been used which generate many spectral lines from a high intensity plasma inside a glass lamp.

• (Electrons are excited to higher energy levels by collisions in the plasma. Photons are emitted when the energy is released.)

• g line - • i line - (used for 0.5 µm, 0.35 µm)

• Brightest sources in deep UV are excimer lasers

€

€

=436 nm

€

=365 nm

€

Kr +NF3 energy⏐ → ⏐ ⏐ KrF → photon emission (1)

• KrF - (used for 0.25 µm, 0.18µm, 0.13 µm)• ArF - (used for 0.13µm, 0.09µm, . . . )• FF - (used for ??)

€

=248 nm

€

=193 nm

€

=157 nm

• Issues include finding suitable resists and transparent optical components at these wavelengths.

B. Wafer Exposure Systems

• Three types of exposure systems have been used.

MaskPhotoresist

Si Wafer

LightSource

OpticalSystem

Contact Printing Proximity Printing Projection Printing

Gap

1:1 Exposure Systems Usually 4X or 5XReduction

• Contact printing is capable of high resolution but has unacceptable defect densities.• Proximity printing cannot easily print features below a few µm (except for x-ray systems).• Projection printing provides high resolution and low defect densities and dominates today.• Typical projection systems use reduction optics (2X - 5X), step and repeat or step and scan mechanical systems, print ≈ 50 wafers/hour and cost $10 - 25M.

5-25µm

B1. Optics - Basics and Diffraction

• Ray tracing (assuming light travels in straight lines) works well as long as the dimensions are large compared to .• At smaller dimensions, diffraction effects dominate.

LightSource

ApertureImage Plane

• If the aperture is on the order of , the light spreads out after passing through the aperture. (The smaller the aperture, the more it spreads out.)

€

€

a). b).

small Plane wave continues due to superposition

Spherical secondary wavelets - same

€

More spherical propagationAperture - fewer wavelets

Huygens-Fresnel principle

Free space

Small aperture

Diffracted LightCollected Light

ImagePlaneAperture

PointSource

Collimating Lens

Focusing Lens

f

d

• A simple example is the image formed by a small circular aperture (Airy disk).• Note that a point image is formed only if , or .

• If we want to image the aperture on an image plane (resist), we can collect the light using a lens and focus it on the image plane.• But the finite diameter of the lens means some information is lost (higher spatial frequency components).

1.22 /f d

€

→ 0

€

f→ 0

€

d→ ∞

• Diffraction is usually described in terms of two limiting cases:• Fresnel diffraction - near field.• Fraunhofer diffraction - far field.

Diffraction

Lost information

A point source does not produce a point image:

Second rings

€

=1.22⋅fd

B2. Projection Systems (Fraunhofer Diffraction)

• These are the dominant systems in use today. Performance is usually described in terms of: • resolution

• depth of focus• field of view• modulation transfer function• alignment accuracy

• throughput

ImagePlane

PointSources d

A

BA'

B'

α

EntranceAperture

• Consider this basic optical projection system.• Rayleigh suggested that a reasonable criterion for resolution is that the central maximum of each point source lie at the first minimum of the Airy pattern.• With this definition,

€

R = 0.61 n sinα

(2)

• The numerical aperture of the lens is by definition, • NA represents the ability of the lens to collect diffracted light.

€

NA ≡n sinα (3)

How to recognize A&B

f

(4)

€

DOF =±

2 NA( )2=± k2

NA( )2

• k2 is usually experimentally determined.

• Thus a 248nm (KrF) exposure system with a NA = 0.6 would have a resolution of ≈ 0.3 µm (k1 = 0.75) and a DOF of ≈ ± 0.35 µm (k2 = 0.5).

€

ResolutionR =1.22fd

=1.22f

n 2f sinα( )=

0.61nsinα

Numerical ApertureNA=nsinα

∴R =0.61NA

=k1NA

€

Depth of focus DOF= =/2(NA)2=k2/2(NA)2 NA DOF , but R Flat topology!

Ex. KrF, 248nm, NA=0.6, k1=0.75, k2=0.5, R=0.31m, DOF =0.34 m

• k1 is an experimental parameter which depends on the lithography system and resist properties (≈ 0.4 - 0.8). • Obviously resolution can be increased by:

• decreasing k1 • decreasing

• increasing NA (bigger lenses)

• However, higher NA lenses also decrease the depth of focus.

Parameters of Projection SystemsResolution and Numerical Aperture

Projection Immersion lithography

9

10

Immersion lithography

• 45-nm node (ASML)

11

Hyper-NA Immersion Litho Tool

Renesas process-technology roadmap.

12

• Modulation Transfer Function (MTF) – a measure of the contrast that can be recognized by the resist.

€

MTF = IMAX −IMINIMAX + IMIN (6)

Intensityat Mask

Intensityon Wafer

0

1

0

1IMAX

IMIN

Aperture

LightSource

Condenser Lens

Objective orProjection Lens

Photoresist on Wafer

Mask

Position Position

-> 0.5 for feature recognition by PR

MTF depends on the feature size and on the spacial coherence of the light source

S = light source diameter/condenser diameter =s/d = NAcondenser optics/NAprojection optics

MTF applies to coherent light (steppers have partly coherent illumination).

Separated large structures 4-5x larger

Ideal mask aerial image Structures closer: Due to partial overlapping of two airy discs Resist has t o respond to

the light intensity

MTF=1

• Finally, another basic concept is the spatial coherence of the light source.

LightSource MaskCondensor

Lens

s d

• Practical light sources are not point sources. the light striking the mask will not be plane waves.

€

∴

• The spatial coherence of the system is defined as

€

S= light source diameter

condenser lens diameter=sd

(7)

or often as

€

S= NA condenser

NA projection optics(8)

• Typically, S ≈ 0.5 to 0.7 in modern systems.

Spatially coherent source

Ideal point source The same

angle of arrival

Modulation Transfer FunctionPlane wave

Various angles

not point

S

Modulation Transfer Function

S=light source diameter/condenser diameter=s/d=NAcondenser optics/NAprojection optics

Diffraction effect MTF

Lower contrast in the aerial image

Less coherent light Improvement for

very small features

s≈0.5-0.7 for s->0 optical intensity decreases

B3. Contact and Proximity Systems (Fresnel Diffraction)

• Contact printing systems operate in the near field or Fresnel diffraction regime.• There is always some gap g between the mask and resist.

WaferResist MaskAperture

Incident Plane Wave

Light Intensityat Resist Surface

g

W

• The aerial image can be constructed by imagining point sources within the aperture, each radiating spherical waves (Huygens wavelets). • Interference effects and diffraction result in “ringing” and spreading outside the aperture.

small

Ringing effect by incoherent light

Some resist exposure

Ringing due to constructive and destructive interference

W=feature size

• Fresnel diffraction applies when

€

< g< W2

(9)

• Within this range, the minimum resolvable feature size is

€

Wmin ≈ g (10)

• Thus if g = 10 µm and an i-line light source is used, Wmin ≈ 2 µm.

• Summary of wafer printing systems:

WaferResist MaskAperture

Incident Plane Wave

Light Intensityat Resist Surface

W

Contact

ProximityProjection

Separation Dependson Type of System

Contact and Proximity Systems

The aerial image here can be calculated if <g<W2/

The minimum feature size that can be resolved here is Wmin(g)1/2

365 nm 10 m W ≈ 2µm

For contact printing -> 0.1 µm (not used!)

C. Photoresists

hydrocarbon-based materials: they change their chemical properties upon illumination by photons

• Resists are organic polymers that are spun onto wafers and prebaked to produce a film ≈ 0.5 - 1 µm thick.

Negative(older –resolution limited by swelling)- more soluble when not exposed. Positive – more soluble when exposed. Important parameters:

• Sensitivity – how much time is reqd. for changes [mJcm-2] (Ex. 100 mJcm-2 for g line and i-line resists, newer down to 20 mJcm-2 .

• Resolution - exposure system rather not the resist limits the Resolution

•Robustness to etching.

Photoresists for g-line and i-line: a hydrocarbon inactive resin, a (hydrocarbon) photoactive compound(PAC), and a solvent: PAC replaced by a Photo Acid Generator (PAG) to act as a chemical amplifier for DUV P.R.

G-line and I-line resists : Diazonaphtoquinones DNQ+resin.

g-Line and i-Line Resists

• Generally consist of 3 components:• Inactive resin• Photoactive compound (PAC)• Solvent - used to adjust viscosity

• After spinning and baking resists ≈ 1:1 PAC and resin.• Diazonaphthoquinone or DNQ resists are commonly used today for g-line and i-line resists.

CH2

CH3 CH3

CH2 CH2

OHOH

• The base resin is novolac a long chain polymer consisting of hydrocarbon rings with 2 methyl groups and 1 OH group attached. Novolac –inactive resin : would dissolve in a developer 15nm/s

Two methyl groups &OH

• The PACs in DNQ resists are often diazoquinones. The photoactive portion is above the SO2. • Diazoquinones are insoluble in typical developers and reduce the dissolution rate of unexposed resists to ≈ 1 - 2 nm sec-1.

O

N2

SO2

CH3CCH3

⇒O

N2

R

O

N2

R

O

N2

R

+

O

N2

R

+

C

WolffRearrangement

OH

O

N2

R

+

C

• After exposure to light, the PAC component in DNQ resists undergoes a transformation (Wolff rearrangement) into carboxylic acid which is soluble in the developer (basic solution).

Photoactive compound in soluble developers it solubility of PR 1-2nm/s

light

Easy dissolution in a basic developer NaOH/H2O , TMAN etc.

100-200 nm/sec

Exposure

Stabilizes itself

In water carboxylic

DUV Resists• g-line and i-line resists have maximum quantum efficiencies < 1 and are typically ≈ 0.3. • Chemical amplification can improve this substantially.• DUV resists all use this principle. A catalyst is used. • Photo-acid generator (PAG) is converted to an acid by photon exposure. Later, in a post exposure bake, the acid molecule reacts with a “blocking” molecule on a polymer chain, making it soluble in developer AND REGENERATING THE ACID MOLECULE∴ catalytic action ∴ sensitivity is enhanced.

PAG

INSOL INSOL

Exposure

INSOL INSOL

Acid

Post ExposureBake (PEB)

SOL INSOL

Acid

SOL SOL

AcidAcid

a) b)

c)d)

Polymer Chain Polymer Chain

Polymer Chain Polymer Chain

Acid

These molecules make the resist insoluble in developer.

Photo Acid GeneratorT< 1 °C

120 °C TEMPERATURE CONTROL

Acid changes the PR properties in baking – reacts with INSOL and converts it into soluble molecules in alkali developers.

Catalytic behavior of acids - reaction repeated (20-40mJcm-2)

Soluble in aqueous alkaline developer

Sensitivity (also easily affected by contaminations, dust etc.)

5x more than DNQ

Acid labile group

Basic Properties of Resists

• Two basic parameters are used to describe resist properties, contrast and the critical modulation transfer function or CMTF.

Fraction of Resist Remaining

0.25

0.5

0.75

1.0

0

Exposure Dose (log)1 10 100

Exposure Dose (log)1 10 100

0.25

0.5

0.75

1.0

0

Positive Resist

Negative Resist

D0Df D0

Df

• Contrast is defined as

€

γ= 1

log10D f

D0

(11)

• Typical g-line and i-line resists achieve values of 2 - 3 and Df values of about 100 mJ cm-2. DUV resists have much higher values (5 - 10) and Df values of about 20 - 40 mJ cm-2. DUV: better contrast and better sensitivity.

Contrast allows distinguishing light and dark areas on the mask. DUV resists have better contrast and better sensitivity because of chemical amplification.

Affected by processing conditions also

CONTRAST = SLOPE

.

0.25

0.5

0.75

1.0

0

Exposure Dose

Position

Df

D0

Areal Image

• The aerial image and the resist contrast in combination, result in the quality of the latent image produced. (Gray area is “partially exposed” area which determines the resist edge sharpness.)• By analogy to the MTF defined earlier for optical systems, the Critical Modulation Transfer

Function (CMTF) for resists is defined as

€

CMTF resist=D f −D0

D f +D0

=101 / γ −1101 / γ +1

(12)

• Typical CMTF values for g and i-line resists are about 0.4. Chemically amplified DUV resists achieve CMTF values of 0.1 - 0.2 (because of higher sensitivity).• Lower values are better since in general CMTF < MTF is required for the resist to resolve the aerial image.

Resists

P

P WellN Well

PNP+ P+ N+ N+

Ideal Aerial Image

Exposing Light

• There are often a number of additional issues that arise in exposing resist.

• Resist is applied as a liquid and hence "flows" to fill in the topography.

∴Resist thickness may vary across the wafer. This can lead to under or over exposure in some regions and hence linewidth variations.

Exposure Uniformity Degradation

z0EII α−=

Where z is the depth and α is the optical absorption. ( More photons absorbed at the surface)

Bleaching effect : Absorption of incoming photons decreases in already absorbed regions. It allows for uniformity improvements.

Non uniform resist thickness (hills and valleys).

•Overexposure (bigger size)

•Underexposure

Light intensity decreases

Aerial Image

Exposing Light

P

P WellN Well

PNP+ P+ N+ N+

A A AB B

Resist

ReflectingSubstrate

Aerial Image

• Reflective surfaces below the resist can set up reflections and standing waves and degrade resolution.• In some cases an antireflective coating (ARC) can help to minimize these effects. Baking the resist after exposure, but before development can also help.

(Photo courtesy of A. Vladar and P. Rissman, Hewlett Packard.)

Post-baking (before development) “scalloped edges” due to diffusion of PAC or PAG

Light reflected by the layer below PR .

Use Anti-Reflective coating to prevent SANDING WAVE.

Standing Wave During Exposure

Surface cleaning and/ordehydration baking

Prepares surface forphotoresist application

HMDS ApplicationAdhesion promoter typically

spun on the wafer

Resist ApplicationResist spun on wafer, typically

3000-6000 rpm, forming an ≈ 0.5 µm resist layer

Pre-bake resist Typically 10 - 30 min @90 - 100 ˚C

Align wafer,expose resist

Typically aligned at eachexposure site, exposed at

150 mJ cm-2

Post exposure bakeSometimes used to minimize

standing waves in resistTypically 10 min @ 100˚C

Resist developTypically 30 - 60 sec @ room T

using spray or immersion

Post-bake resistTypically 10 - 30 min @

100 - 140 ˚C

Typical resist process:

(Smaller dose forDUV resists)

(Required forDUV resists)

(g and i-line resist parameters)

SubWavelength Lithography

• Beginning in ≈ 1998, chip manufacturers began to manufacture chips with feature sizes smaller than the wavelength of the light used to expose photoresist.• This is possible because of the use of a variety of “tricks” - illumination system optimization, optical pattern correction (OPC) and phase shift mask techniques.

(From Synopsis Website - www.synopsis.com)