Basic Concepts

C05 - 1Virginia Tech

Basic Concepts

Antireflective coating is used to prevent reflections from the chrome coming back into the resist– Occasionally AR coatings are deposited on wafers

also Develop the resist and etch to remove the metal

– We get good dimensional control because the Cr is very thin (~80nm)

It is critical that the areas beneath where the Cr is removed be highly transparent at the wavelength of the light used in the wafer exposure system.

Masks (reticules) for steppers (step and repeat systems) are 4x to 5x larger than what is printed– Relaxes minimal feature requirements on mask

Masks for steppers print usually only one or two die at a time; any defect in the mask gets reproduced for every die!


Reflectivity

At the interface of two bulk layers

2

21

21

nn

nnR

http://www.mellesgriot.com/products/optics/images/fig5_12.gif


Antireflectivity Coatings

– For /4 thick films

Ideal index of refraction for antireflective coating is √(nairnglass)

2

2

filmglassair

filmglassair

nnn

nnnR


Basic Concepts

We generally separate lithography into three parts– The light source– The exposure system– The resist

The exposure tool creates the best image possible on the resist (resolution, exposure field, depth of focus, uniformity and lack of aberrations)

The photoresist transfers the aerial image from the mask to the best thin film replica of the aerial image (geometric accuracy, exposure speed, resist resistance to subsequent processing)


Light Source

Historically, light sources have been arc lamps containing Hg vapor

A typical emission spectra from a Hg-Xe lamp

Low in DUV (200-300nm) but strong in the UV region (300-450nm)


Light Source

To minimize problems in the lens optics, the lamp output must be filtered to select on of the spectral components.

Two common monochromatic selections are the g-line at 436 nm and the i-line at 365 nm.

The i-line stepper now dominates the 0.35 m market


Light Sources

For 0.18 and 0.13, we use two excimer lasers (KrF at 248 nm and ArF at 193 nm)

These lasers contain atoms that do not normally bond, but if they are excited the compounds will form; when the excited molecule returns to the ground state, it emits

These lasers must be continuously strobed (several hundred Hz) or pulsed to pump the excitation

Can get several mJ of energy out Technical problems have been resolved for KrF and

these are used for 0.25 and 0.18 m ArF is likely for 0.18 and 0.10 m; technical problems

remain


Exposure System

There are three classes of exposure systems– Contact– Proximity– Projection


Exposure System

Contact printing is the oldest and simplest The mask is put down with the Cr in contact

with the wafer This method

– Can give good resolution– Machines are inexpensive– Cannot be used for high-volume due to

damage caused by the contact– Still used in research and prototyping

situations


Wafer Exposure Systems

Proximity printing solves the defect problem associated with contact printing– The mask and the wafer are kept about

5 – 25 m apart – This separation degrades the resolution– Cannot print with features below a few

microns– The resolution improves as wavelength

decrease. This is a good system for X-ray lithography because of the very short exposure wavelength (1-2 nm).


Wafer Exposure Systems

For large-diameter wafers, it is impossible to achieve uniform exposure and to maintain alignment between mask levels across the complete wafer.

Projection printing is the dominant method today– They provide high resolution without the defect

problem– The mask (reticule) is separated from the wafer and

an optical system is used to image the mask on the wafer.

– The resolution is limited by diffraction effects– The optical system reduces the mask image by 4X to

5X– Only a small portion of the wafer is printed during

each exposure– Steppers are capable of < 0.25 m– Their throughput is about 25 – 50 wafers/hour


Optics Basics

We need a very brief review of optics If the dimensions of objects are large

compared to the wavelength of light, we can treat light as particles traveling in straight lines and we can model by ray tracing

When light passes through the mask, the dimensions of objects are of the order of the dimensions of the mask

We must treat light as a wave


Optics Basics

Diffraction occurs because light does not travel in straight lines

Pass a light through a pin-hole; we see that the image is larger than the hole

This cannot be explained by ray tracing


Diffraction of Light



The Huygens-Fresnel principle states that every unobstructed point of a wavefront at a given time acts as a point source of a secondary spherical wavelet at the same frequency

The amplitude of the optical field is the sum of the magnitudes and phases

For unobstructed waves, we propagate a plane wave

For light in the pin-hole, the ends propagate a spherical wave.


Young’s Single Slit Experiment

sin = /d

http://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.htmlhttp://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.html


Amplitude of largest secondary lobe at point Q, Q, is given by:

Q = (A/r)f()d

where A is the amplitude of the incident wave, r is the distance between d and Q, and f() is a function of , an inclination factor introduced by Fresnel.

http://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.htmlhttp://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.html


Young’s Double Slit Experiment

http://micro.magnet.fsu.edu/optics/lightandcolor/interference.htmlhttp://micro.magnet.fsu.edu/optics/lightandcolor/interference.html


Basic Optics

This diffraction “bends” the light Information about the shape of the pin hole is

contained in all of the light; we must collect all of the light to fully reconstruct the pattern

The following diagram shows how the system works

Note that the focusing lens only collects part of the diffraction pattern

The light diffracted at higher angles contains information about the finer details of the structure and are lost


Basic Optics


Basic Optics

The image produced by this system is


Basic Optics

The diameter of the central maximum is given by

Note that you get a point source only if d

light ofh wavelengtλ

length focal f

diameter lens focusing

22.1 maximum central ofDiameter

dd

f


Basic Optics

There are two types of diffraction– Fresnel, or near field diffraction– Fraunhofer, or far field diffraction

In Fresnel diffraction, the image plane is near the aperture and light travels directly from the aperture to the image plane (see Figure 5-4)

In Fraunhofer diffraction, the image plane is far from the aperture, and there is a lens between the aperture and the image plane (see Figure 5-6)

Fresnel diffraction applies to contact and proximity printing while Fraunhofer diffraction applies to projections systems

There are powerful simulations systems for both cases


Fraunhofer Diffraction

We define the performance of the system in terms of– Resolution– Depth of focus– Field of view– Modulation Transfer Function (MTF)– Alignment accuracy– throughput



Imagine two sources close together that we are trying to image (two features on a mask)

How close can these be together and we can still resolve the two points?

The two points will each produce an Airy disk (5-7)

Lord Rayleigh suggest that we define the resolution by placing the maximum from the second point source at the minimum of the first point source



With this definition, the resolution becomes

For air, n=1 is defined by the size of the lens, or by an

aperture and is a measure of the ability of the lens to gather light

light diffracted theof angle half maximum

lens andobject ebetween th material theof refraction ofindex

sin

61.0

sin2

22.122.1

n

nfn

f

d

fR



This is usually defined as the numerical aperture, or NA

This really is defined only for point sources, as we used the point source Airy function to develop the equation

We can generalize by replacing the 0.61 by a constant k1 which lies between 0.6 and 0.8 for practical systems

NAk

NAR

nNA

1

61.0

sin



From this result, we see that we get better resolution (smaller R) with shorter wavelengths of light and lenses of higher numerical aperture

We now consider the depth of focus over which focus is maintained.

We define as the on-axis path length difference from that of a ray at the limit of the aperture. These two lengths must not exceed /4 to meet the Rayleigh criterion


Depth of Focus


Depth of Focus

From this criterion, we have

For small

cos4/

22114/

22

2222

22sin

NAk

NADOF

NAf

d



From this we note that the depth of focus decreases sharply with both decreasing wavelength and increasing NA.

The Modulation Transfer Function (MTF) is another important concept

This applies only to strictly coherent light, and is thus not really applicable to modern steppers, but the idea is useful



Because of the finite aperture, diffraction effects and other non-idealities of the optical system, the image at the image plane does not have sharp boundaries, as desired

If the two features in the image are widely separated, we can have sharp patterns as shown

If the features are close together, we will get images that are smeared out.


Modulation Transfer Function



The measure of the quality of the aerial image is given by

The MTF is really a measure of the contrast in the aerial image

The optical system needs to produce MTFs of 0.5 or more for a resist to properly resolve the features

The MTF depends on the feature size in the image; for large features MTF=1

As the feature size decreases, diffractions effects casue MTF to degrade

MINMAX

MINMAX

II

IIMTF


Change in MTF versus Wavelength


Contrast and Proximity Systems

These systems operate in the near field or Fresnel regime

Assume the mask and the resist are separated by some small distance “g”

Assume a plane wave is incident on the mask Because of diffraction, light is bent away for

the aperture edges The effect is shown in the next slide Note the small maximum at the edge; this

results from constructive interference Also note the ringing As a result, we often use multiple wavelengths


Fresnel Diffraction


Fresnel Diffraction

As g increases, the quality of the image decreases because diffraction effects become more important

The aerial image can generally be computed accurately when

where W is the feature size Within this regime, the minimum resolvable

feature size is

2Wg

gW min


Depth of Focus

http://www.research.ibm.com/journal/rd/411/holm1.gif


Summary of the Three Systems

Basic Concepts

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