MSE-630

Lithography

Topics:

•Wafer exposure systems

•Photoresists

•Manufacturing Methods & Equipment

•Measurement Methods

MSE-630

Projected Lithography Requirements

MSE-630

MSE-630

Projection systems

Contact Printing:•Limited diffraction effects

•Inexpensive

•Contact between mask & resist results in damage – low yield

•Oldest & simplest

Proximity Printing:•Mask/wafer separated 5-25 m

•Separation results in poor resolution

•Limited to m-sized features

•Both systems require 1X masks

MSE-630

Projection systems

•Produce high resolution w/o defects

•Resolution limited by diffraction effects

•Mask is 4X – 5X image size

•Sub-micron features

•50 wafer/hour throughput

MSE-630

Diffraction effectsLight passing through

aperture, with an opening ≈, diffracts,

creating a larger image than that on the

mask

Light travels as a plane wave in free space. The spherical wavelets combine to for a uniform front. When it passes through an aperture, the waves from the limited number of wavelets superimpose, spreading out in all directions.

Smaller openings mean greater diffraction

MSE-630

The light that diffracts at the highest angels carries detailed information about the shape of the aperture,

is not collected passing through the focusing lens

and is lost.

Projected intensity of light through a circular aperture. Rings around center bright spot result from diffraction

d

fD

22.1

D = diameter of central disk

f = focal length

d = objective lens diameter

MSE-630

A Fourier series is used to describe the path of light:

11

11

)(211

''

,(),(

''

0

1),(

),()(

yxtFffe

writeweshorthandinz

yfand

z

xf

areasopaquein

areasclearinyxt

where

dxdyeyxtyxe

yx

yx

yfxfi yx

The intensity is given by: I(fx,fy) = |(fx,fy)|2 = |

F[t(x1,y1)]|2

MSE-630

Resolution

sin

sin

61.0

)sin2(

22.122.11

nNA

refractionofindexn

NAk

nfn

f

d

fR

MSE-630

Depth of Field

222

22

2

2sin

2211

4

,

cos4

NAk

NADOF

NAf

d

smallisif

The Rayleigh criteria for depth of focus states that the path difference for a ray on the center line and one coming from the edge of the aperture should not differ by more than /4.

MSE-630

Example

Estimate the resolution and depth of focus of an excimer laser stepper using KrF light source ( = 248 nm) and NA=0.6 Assume k1 = 0.75 and k2 = 0.5.

Solution:

R = k1*/NA = 0.75(0.248/0.6) = 0.31 nm

DOF = ± k2*/NA2 = ±0.5(0.248/(0.6)2) = ±0.34 m

MSE-630

Modulation Transfer Function (MTF)

minmax

minmax

II

IIMTF

The MTF is a measure of the quality of contrast between features. As features move closer together, diffraction affects cause their Airy disks to begin to overlap, changing the degree of intensity between the two features.

Generally, a MTF>0.5 is needed. Smaller values limit the minimum feature size

MSE-630

Aerial images produced by contact printing (dashed line) proximity, and projection systems. g=0 in contact system and ~25 microns in proximity system.

The quality of the image decreases as the mask is removed from the surface, with gap size g. The image can be calculated when the gap falls between

< g < W2/2where W = mask opening width

The minimum resolvable feature size is: Wmin ≈ √(g)

MSE-630

Simulated aerial image. Shading corresponds to optical intensity in the aerial image. The black borders correspond to the mask image that is being printed. The exposure system simulated has an NA = 0.43, partially coherent g-line ( = 436nm). Min feature size is 1 m.

Left: same example as above except feature size has been reduced to 0.5 m. Note the much poorer image quality

Below: Same example, but wavelength has been reduced to 0.365 nm and NA has been increased to 0.5. Note improvement in image quality

MSE-630

Light CoherencyA perfect point source creates parallel beams after passing

through the condensor lens. A light source with finite size

causes light to strike mask at a variety of angles

Coherent Partially Coherent

A definition of spatial coherence of light sources is:

S = light source diameter/condenser lens diameter = s/d

Or S = NAcondenser optics/NA projection optics

As s becomes more incoherent, MTF degrades for

larger features but improves for smaller ones.

MSE-630

Photoresists

Typical process for g-line and i-line resists

•Resist changes chemical properties when exposed to light

•Resist may be positive or negative. In positive resists, the exposed areas dissolve when developed

•Resist is poured on to wafer, then wafer is spun. Viscosity is controlled by solvents, which evaporate

•Following patterning and developing, resist is baked to increase hardness

MSE-630

Photoresists

Basic structure of diazoquinone, a photo active compound (PAC) in positive resists

Sensitivity is a measure of the light required to expose a resist. In Deep UV resists (DUV), this is 20-40

mJ/cm-2. Lower sensitivity gives higher contrast and increased processing latitude.

The above diagram illustrates how diazoquinone changes when exposed

to light. The final molecule (bottom left) is carboxylic acid, which is

soluble in common developers (e.g., KOH, NaOH, TMAH (tetramethyl

ammonium hydroxide))

MSE-630

As wavelengths get shorter, photoresist has to be reformulated to have the desired reactivity and sensitivity

In DUV resists, incoming photons react with a photo-acid generator (PAG) molecule, creating an acid molecule that acts as a catalyst to make resist molecule soluble. This

process can repeat tens or hundreds of times during the post-exposure

bake (PEB).

Sensitivity of these resists is ~ 20-40 mJ/cm-2

This scheme is being exploited in a new generation of resists designed to

work at short wavelengths.

MSE-630

Properties and Characterization of Resists

Idealized contrast curves for positive and negative resists

Resist is characterized by its

contrast and critical modulation transfer

function (CMTF)

Contrast is its ability to distinguish light from dark areas in the aerial image

Resists with high contrast can sharpen a poor aerial image

o

f

Q

Qlog

1 Qo = dose at which exposure begins to have

an effect

Qf = dose at which exposure is complete

MSE-630

The MTF was used to measure the “dark” versus “light” intensities in the aerial image. A similar

quantity is used for the resist, called the Critical MTF. It is the minimum optical transfer function

needed to resolve a pattern in the resist.

110

1101

1

0

of

fresist QQ

QQCMTF

Example of how the quality of the aerial image and the resist contrast combine to produce the resist edge profile. The left side shows a sharp aerial image and steep resist edges. The example on the right shows a

poorer aerial image and the resulting gradual edges on the resist profile.

MSE-630

Resist Exposure Issues

Resist coatings are:

•Non-uniform thickness

•Not exposed simultaneously

•Light absorption falls off exponentially with increasing depth:I = Ioexp(-z)

Highly reflective substrates create standing waves that

systematically over/underexpose resist, resulting in “wavy” lines:

MSE-630

Mask Engineering: Optical Proximity Correction

Because high frequency detail is lost due to

diffraction defects and apertures and lenses are round, square details on mask are lost on aerial

image (left). OPC adjusts the image on the right to provide greater detail in

the projected pattern

MSE-630

Mask Engineering: Phase Shifting

In the image at left, diffraction effects result

in a loss of contrast where the waves

overlap.

On the right, a material is added to the mask to cause the light to shift phase 180o, thereby

canceling the effect of overlap.

Phase shifting can be used to improve the quality of the aerial image or to improve the depth of focus of the exposure system at constant resolution by

using a lower NA system

MSE-630

Wafer Exposure SystemsThe schematic at left illustrates a scanning

system for transferring information to the

wafer. This requires a 1X mask.

Modern systems use a 5X (or larger mask)

and combine scanning with a stepper to

systematically cover small portions of the

wafer.

MSE-630

Techniques to improve image quality

In the Kohler illumination system (left), light is focused at the

entrance pupil of the projection lens, which captures the diffracted

light from any features on the mask equally well

Off-axis illumination captures some of the higher-order

diffracted light which was lost in the normal illumination process

MSE-630

Measurement Methods

Mask inspection measurements read

the mask and compare it to another mask or a mask in a

design database

Typical mask flaws include:

•Opaque defects: Cr where it should not be

•Clear defects: Cr isn’t where it should be

•The actual size of the feature on the mask, which is influenced by the size of the e-beam spot used to write the mask (typ. ~0.125 m)

•Proximity effects from electron backscattering in the resist resulting in distortion

MSE-630

Test Structures

65

43

2ln

I

Vs

This is an example of a typical test

structure built into the edges of a

chip/sheet.

It is used to extract the sheet resistance

The geometry is chosen to define one square of the

material (labeled s)

A current, I5-6 is applied, and voltage, V3-4, is read at terminals 3 & 4. The sheet resistance is then:

Given the sheet resistance, the line width can be calculated

from: 32

51

V

ILW s