VLSI Technology Lecture 12

Devesh Chandra Guest Faculty

MNIT Jaipur

Lithography Introduction & Principles Optical Lithography

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Lithography

Introduction and

Principles

Instruments &

Lithography Exposure System

Optical Lithography

Electron Lithography

X- Ray Lithography

Ion Lithography

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Introduction

Why do we need lithography ? The ability to print patterns with sub-micron

features and to place those patterns on a silicon substrate with better than 0.1 um precision

The concept is simple A light sensitive photo-resist is spun onto wafer

forming thin layer on the surface. The resist is then selectively exposed by shining light

through mask, which contains the pattern information Resist is then developed which completes pattern

transfer from mask to wafer The resist may then be used to as mask to etch

underlying films or for ion implantation

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The concept is simple, but actual implementation is expensive and very complex

The following aspects are considered in such systems:- resolution, exposure field, placement accuracy, throughput , defect density.

Resolution Ever increasing demand for smaller device structures

Exposure Field Ever increasing chip sizes

Placement accuracy

Mask Layers should be carefully aligned with respect to the existing patterns already in the wafer

Throughput Competitive nature of semiconductor industry

Defect density - do -

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Process flow in Optical Lithography System

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Optical Lithography Process

The patterns which comprises of various layers in IC are designed using the CAD tool. The facilities offered these tools are Can design the chips with multi-million

components

List and design of previous works are available

Routing & wiring is done

DRC

Circuit and system level simulation are possible to study the performance

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Once the design is complete, it is transferred to the fabrication facility

Masks - The information for each mask level is transferred to mask making machine Mask machine could be laser pattern generator or

electron beam generator The pattern of the mask is written on a mask blank

using scanning beam or laser Mask is made up of fused silica and is covered with a

thin layer (80nm) of Chromium layer and a layer of photo-resist

Anti reflecting coating is present between chromium and photo-resist to prevent reflection from the chrome layer

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The electron or laser beam exposes the resist, which is then developed and used as etch mask to transfer pattern into the chrome

The chrome layer is finally dry / wet etched

Tight dimensional control is maintained

Once the chrome layer is etched, the photo-resist is removed

The fused silica substrate is highly polished surface so light is not scattered, when it passes through the chrome and has small thermal expansion coefficient.

UV absorption by SiO2 due to trace impurities.

Mask is usually prepared 4 X 5 times larger than the features actually desired.

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Lithography System ( Light source)

Higher resolution lithography requires shorter wavelength photon

Arc Lamps Hg vapor inside a sealed glass envelope Two conducting electrodes are separated by several

mm length. An arc is struck between the electrode by applying

high voltage which ionise the gas (kV), and behaves as plasma

In this lamp plasma is conducting and consists of ions, electrons and neutral species

Initial pressure is around 1 atm, but later it increases to 20-40 atm 9

Light emission occurs due to two process

Black body radiation ( free electrons in the arc). It is in the deep UV region and mostly absorbed by glass envelope of the lamp

Emitted light from Hg atoms. This is emitted in UV range

The complex optical systems are easier to design if they focus on single wavelength. The unwanted wavelengths are filtered out.

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Two commonly used lines are 436 nm (g line) and 365 nm (i-line).

I line dominated production for the 0.35 um generation of technology.

New light sources are required for beyond this resolution

KrF (248 nm)

ArF (193 nm)

These light sources are generated using excimer laser systems and chemical mix of Kr, NF3

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Wafer Exposure System

There are three general class of the optical wafer exposure tools

Contact , proximity and projection systems

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Schematic of different types of Wafer Exposure Systems

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Contact Printing Proximity Printing Projection Printing

Oldest and simplest

Mask is placed with chrome side down in direct contact with resist layer of the wafer

Mask and wafer are separated by 5 25 um

Mask is separated from wafer. An optical system is used to image the mask on the wafer

Alignment of mask is done prior to exposure using microscopes

Can produce high resolution printing as diffraction effects are minimized

Separation degrades the resolution due to diffraction effects

Resolution is limited by diffraction effects.

Machine are relatively inexpensive

Can be used in X Ray lithograhy

Costly systems

Not suitable for high volume manufacturing Hard contact between mask and resist may damage of mask and resist layer, hence high defect densitites

Proximity printing solves the defect issues

Without defect problems associated with contact printing. Suitable for bulk processing (25 50 ) wafers per hour.

1X Mask 1 X Mask 4X 5X Mask 14

Optics Basic Ray Optics and Diffraction

Ray Optics

Wave Optics

Diffraction

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Simple Diffraction Effect. Light passes through a narrow aperture. The image formed covers a much

larger area than can be explained by ray tracing 16

Propagation of a plane wave in free space and through small aperture ( Huygens Fresnel

Principles) 17

Diffraction

Diffraction can be thought as the bending of the light, when it passes through the aperture carries with it the information of shape and size of the aperture. (Example aperture on the mask to be printed on the resist)

The problem is that information spreads out in the space because of diffraction and it must be collected to convey perfect information about the aperture to the resist

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Qualitative Example of small aperture being imaged

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Because of the finite size of the lens only a portion of the total light is collected and focused on the resist.

The figure next shows the actual image produced by such aperture on the resist.

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Image Intensity of circular aperture in the image plane ( Fraunhoffer diffraction pattern). 2D

image 21

Fresnel and Fraunhoffer diffraction

Fresnel Fraunhoffer

It is also called near field diffraction Also known as far field diffraction

The image plane is close to the aperture The image plane is far from the aperture

The light wave travels directly from aperture to the image plane

No intervening lens Lens is generally placed in between the aperture and image plane for focus

Contact & Proximity exposure system Projection Systems

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Resolutions & Imaging

The limit of resolution (or resolving power) is a measure of the ability of the objective lens to separate in the image adjacent details that are present in the object. It is the distance between two points in the object that are just resolved in the image.

The resolving power of an optical system is ultimately limited by diffraction by the aperture.

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Resolution and

Diffraction Concept

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For resolution to occur, at least the direct beam and the first-order diffracted beam must be collected by the objective. If the lens aperture is too small, only the direct beam is collected and the resolution is lost.

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Consider a grating of spacing d illuminated by light of wavelength , at an angle of incidence i.

The path difference between the direct beam and the first-order diffracted beam is exactly one wavelength, . So, ( d sin i + d sin = ) where 2 is the angle through which the first-order beam is diffracted. Since the two beams are just collected by the objective, i = , thus the limit of resolution is,

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Numerical Aperture

The numerical aperture of a microscope objective is a measure of its ability to resolve fine specimen detail. The value for the numerical aperture is given by,

Numerical Aperture (NA) = n sin where n is the refractive index and equal to 1 for air and is the half angle subtended by rays entering the objective lens.

Numerical aperture determines the resolving power of an objective, the higher the numerical aperture of the system, the better the resolution

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Low numerical aperture Low value for a Low resolution

High numerical aperture High value for a High resolution 28

NA and spread in Airy Disk

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When light from the various points of a specimen passes through the objective and an image is created, the various points in the specimen appear as small patterns in the image. These are known as Airy discs. The phenomenon is caused by diffraction of light as it passes through the circular aperture of the objective.

Airy discs consist of small, concentric light and dark circles. The smaller the Airy discs projected by an objective in forming the image, the more detail of the specimen is discernible. Objective lenses of higher numerical aperture are capable of producing smaller Airy discs, and therefore can distinguish finer detail in the specimen.

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Rayleigh Criterion

The limit at which two Airy discs can be resolved into separate entities is often called the Rayleigh criterion. This is when the first diffraction minimum of the image of one source point coincides with the maximum of another.

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Un-resolvable Rayleigh Criterion Resolvable

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Fraunhofer Diffraction

Consider the situation when we have two point sources close together that we are trying to image. These could be two small adjacent features on the mask we are trying to print in the resist on a wafer.

How close together can they be and still be resolved in the image plane

Image produces by the two point source will each be an Airy disk.

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Resolving power of a lens when two point sources are to be separated in the image

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R is the resolution of the lens, n is the index of refraction of the material between object and the lens (1 for air), alpha is the half of the maximum of the diffracted light that can enter the lens (acceptance angle of the lens)

NA is the numerical aperture

In lithographic system 0.61 factor is replaced by k1, as 0.61 was derived for point source ,but in real situation the mask could take any shape. Actual k1 values achieved is 0.6 to 0.8

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Depth of focus

The effect of the focus on a projection lithography system is a critical factor in understanding and controlling lithographic process

As feature size decreases, their sensitivity to focus errors increases

DOF can be thought of as the range of the focus errors that a process can tolerate and still give the acceptable lithographic results

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The classic Rayleigh criterion for depth of focus is based on the visible change in the image, this change can be insignificant or catastrophic for microcircuit manufacturing

In lithography, a shift in focus produces two major changes to the final result: Photo-resist profile changes

Sensitivity of the process to other processing errors

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Images showing the concept of Depth of Focus (DOF)

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Depth of focus decreases as the numerical aperture increases

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Contact Printing

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Proximity Printing Near Field Fresnel Diffraction

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Projection Printing

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Images produced by three types of optical lithographical tool.

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Modulation Transfer Function

The modulation transfer function is as name suggest is a measure of the transfer of modulation ( or contrast) from the subject to the image. In other words, it measures how faithfully the lens reproduces (or transfers) details from the object to the image produced by the lens.

The Blurring is due to the phenomenon of diffraction. Diffraction is the fundamental optical limit on the image quality and resolution that results from wave nature of light and finite diameter of the lenses.

MTF = (maximum intensity - minimum intensity)/(maximum intensity + minimum intensity)

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[A] is the original image [B] is the image of the test pattern

[C] Line pattern of the original test pattern [D]The line profile of the image of the test pattern

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Modulation and Contrast

Transfer Function

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Modulation Transfer Function (MTF) 48

The concept applies to the incoherent illumination.

Diffraction effects is only important after the light passes through the mask, the optical intensity pattern as the light exits the mask will be almost ideal representation of the mask.

A useful measure of the quality of the image is the MTF that is defined as formula below.

MTF measures the contrast in the image produced by the exposure system

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MTF depends on the feature size in the image.

For large feature size, the resulting image produced by the exposure system has excellent contrast and MTF is unit. As the feature size decreases, diffraction effects cause the MTF to degrade and to finally reach zero when the features are so closely spaced that there is no remaining contrast in the image.

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Modulation Transfer Function versus feature size

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MTF is also affected by the parameter known as spatial coherence of light source.

An ideal point source produces light in which the waves are in phase at all points along the emitted wave-front.

A condenser lens can convert these waves to plane waves all of which strike mask at exactly same angle such source are ideal coherent source

As the physical size of the source increases, light is emitted from volume rather than a point and waves will not be perfectly in phase everywhere

If the same condenser lens is used to convert light to plane waves, such sources will be partially coherent.

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Example of spatially coherent and partially coherent light source

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MTF versus feature size. As s increases (more incoherent source, the MTF degrades for larger

feature size but improves for very small features)

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Lithographic Projection

Lenses

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Photo-resists How photo-resist material differs from other materials

? Interaction of materials with light

Recombination (semiconductor) Phonon interaction (semiconductors) Chemical changes

Photo-resist materials are designed to respond to incident photons by changing their properties when they are exposed to light. The requirement is to maintain the latent image of the impinging photons at least until the resist is developed.

A long lived response to light generally requires the chemical change.

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Almost all resist fabricated today are hydrocarbons, when these materials absorb light, the energy from photons generally breaks chemical bonds, after this the material restructures itself in the another stable form

Positive resist respond to the light by becoming more soluble in the developer solution

Negative resist do the opposite. They become less soluble when they are exposed. Positive resists are widely used in industry today

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Photo-resist used today are liquids at room temperature. They are applied on the wafer by spinning. The thickness of the photo-resist can be controlled by controlling the viscosity of the resist, spin speed. It ranges from 0.6 um to 1um.

Baking step is used to drive off remaining solvent (Pre-bake)

Developing of the photo-resist is done using liquid developer on the wafer

Post bake is done after developing to harden the photo-resist. It improves the ability of the resist to act as etch mask or ion-implantation mask

Finally after etching or ion-implantation, resist is removed in oxygen plasma or chemical stripping

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Photo-resist characteristics

Process compatibility Sensitivity to light i.e. how much light is required to

expose resist. Higher sensitive lights are desired because this reduces the exposure time of the resist. Extremely high sensitive usually is not desired, it tends to make the material unstable, variations with temperature and effect of shot noise

g line, i-line resist is having sensitivity of 100mJ/cm2 and DUV resists often achieve sensitivities of 20 40 mJ /cm2.

Resolution- The quality of resist patterns today is generally limited by the exposure system and not by resist itself.

Resist function:- The term resist describes the need for the photo-resist to withstand etching or ion implantation after the mask pattern is transferred to the wafer

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g and i line photo-resists

It generally consist of three components, an inactive resin (base of material), a photoactive compound, and a solvent.

The most commonly used material for g and i line resist today are diazonapthoquinones (DNQ) materials

A basis resist is generally novolac, a low molecular weight phenol-formaldehyde condensation polymer

Novolak-diazoquinone resist have played a crucial role in the micro-miniaturisation of electronics

They are photographic materials of extremely high resolution, able to define features as small as 0.25 m.

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Basic structure of Diazoquinone, commonly used photoactive compound in photo-resist

Novolac is a polymer material consisting of basic hydrocarbon ring with methyl groups and 1 OH group attached

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The Photoactive in these resists are often diazonaphthoquinones or DNQ materials. The photo-active part of the compound is the portion above SO2.

The role of PAC is to inhibit the dissolution of the resist material in the developer.

These compounds are insoluble in typical developer and dissolution rate of the resist to approximately 1-2 nm/sec.

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Decomposition process occurring in DNQs upon exposure to light 64

The N2 molecule is weakly bonded in the PAC and the first part of the photochemical reaction involves the breakage of bond.

The PAC structure stabilizes itself by moving carbon outside of the ring with the oxygen atom covalently bonded to it (Wolff rearrangement)

The final ketene molecule transform into carboxylic acid in the presence of water.

This is readily soluble in the TMAH (tetramethyl ammonium hydroxide or KOH or NaOH). Novolac material is also soluble in this developer

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A ketene is an organic compound containing the >C=O=O functional group.

Ketene is also the name for the >C=O=O functional group. Ketene is also a common name for the compound ethenone. Ethenone is the simplest ketene molecule where R and R' are both hydrogen atoms.

This is the typical benzene-ketene reaction 66

Deep UV resists

With shorter wavelength materials have two significant problems

Wavelengths below i-line DNQ absorbs the incident wavelength.

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Mask Engineering

The quality of the aerial image of the can be enhanced by designing better masks

This is also known as wave-front engineering

There are two methods for this OPC (Optical Proximity Correction) : Generally we

lost are the high frequency components of the diffraction pattern. This translates as the rounded rather than square corners .

PSM (Phase shift masks ) in 1982 by Leveson and colleagues

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Optical Proximity Correction In addition to the finite size of the optical

systems, apertures and lenses in the projection systems are circular and not rectangular as in the mask

High frequency components of the diffraction pattern are lost.

This lost information results in aerial image which has rounded rather than square corners

Shortening the ends of narrow linear feature

A perfect image on the mask can from diffraction effect, result in a distorted pattern in the resist

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Mask Patterns with and without OPC [Upper] The corresponding aerial image when OPC used. Dark lines in the bottom patterns indicates difference between mask and aerial image in each case

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Optical Mask attempts to reverse the situation by having the distorted image on the mask that is design to produce a perfect image on the resist.

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Phase shifting Method

This method involves changing the transmission characteristics of the mask in selected areas.

Levenson etal. Suggested the use of phase shifting techniques to improve the resolution of the printed aerial image

The limitation of the designing of the desired pattern on mask by CAD system.

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Phase shifting

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In the previous example a periodic mask with equal lines and spaces (diffraction grating) is used as the mask.

In the example on the right, a material whose thickness and index of refraction are chosen to phase shift the light by exactly 180oC is added to the mask. The thickness of this layer is given by

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Lithography

Introduction and

Principles

Instruments &

Lithography Exposure System

Optical Lithography

Electron Lithography

X- Ray Lithography

Ion Lithography

75

References

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Slide Number Reference

57-67 S. M . Sze, VLSI Technology, Tata McGrawHill

3-22, 26, 31, 33-45, 48, 49, 50-55, 68-74

Plummer, Deal Griffin, Silicon VLSI Technology, Prentice Hall

46, 47 http://photo.net/learn/optics/mtf/

24, 25

http://www.doitpoms.ac.uk/tlplib/optical-microscopy/resolution.php

38 http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

28,29,30,32 http://www.microscopyu.com/articles/superresolution/diffractionbarrier.html http://www.microscopyu.com/articles/optics/mtfintro.html

56 Microlithography, Science & Technology, Suzuki, Smith, CRC Press