Prof. Jin-Heong Yim

Functional Polymer/1st Semester, 2006_________________________________________

Part III. Functional Polymers for Semiconductor Applications

Outline of PartPhotoresist for Semiconductor Applications

Introduction of photolithographyPhotoresist Materials for Exposure at 193 nm WavelengthChemically Amplified Resists for F2 Excimer laser Lithography

Prof. Jin-Heong Yim

Motivations

Creation of integrated circuits, which are a major component in computer technology

An extension of photolithography processes are used to create standard semiconductor chips

Play a key role in the production of technically demanding components of advanced microsensors

Used to make adhesives in electronics

Prof. Jin-Heong Yim

History

Historically, lithography is a type of printing technology that is based on the chemical repellence of oil and water.

Photo-litho-graphy: latin: light-stone-writing

In 1826, Joseph Nicephore Niepce, in Chalon, France, takes the first photograph using bitumen of Judea on a pewter plate, developed using oil of lavender and mineral spirits

In 1935 Louis Minsk of Eastman Kodak developed the first negative photoresist

In 1940 Otto Suess developed the first positive photoresist.

In 1954, Louis Plambeck, Jr., of Du Pont, develops the Dycrylpolymeric letterpress plate

Prof. Jin-Heong Yim

A process that involves transferring an integrated

circuit pattern into a polymer film and subsequently

replicating that pattern in an underlying thin

conductor or dielectric film

Microlithography

Prof. Jin-Heong Yim

Comparison of the dimensions of lithographicimages and familiar objects

SEM picture of typical lithographic pattern

Thompson, L. F.; Willson, C. G.; Bowden, M. J. Introduction to Microlithography; 2nd Ed; ACS Professional Reference Book; American Chemical Society; Washington, DC, 1994

How Small Can We Print ?

Prof. Jin-Heong Yim

Roadmap of Semiconductor Technology

10

1.0

0.1

1980 1990 2000 2005

1 M19861.2 4 M

1989 0.8

1 G2001 0.18

64 M19950.35

16 M1992 0.5

256 M1998 0.25

4G20040.13 16 G

2007 0.09

256 K19832.0

64 K19803.0

16 K19775.0

19951985 2010

Contact Printer

g-lineStepper

i-lineStepper

KrF ArF

- C.Rubber- Bisazide

Novolak- DNQ

KrFCAR

ArFCAR

F2, EUV, X-Ray

F2, EUV, X-RayCAR

Resolution

R = kλ

NA

Prof. Jin-Heong Yim

G-lineI-line

DUV

Thompson, L. F.; Willson, C. G.; Bowden, M. J. Introduction to Microlithography; 2nd Ed; ACS Professional Reference Book; American Chemical Society; Washington, DC, 1994

Light intensity(output) of Hg-lamp as a function of wavelength

Prof. Jin-Heong Yim

Photolithography

In photolithography, the pattern is created photographically on a substrate (silicon wafer)

Photolithography is a binary pattern transfer: there is no gray-scale, color, nor depth to the image

This pattern can be used as a resist to substrate etchant, or a mold, and other forms of design processes

The steps involved are wafer cleaning, photoresistapplication, soft baking, mask alignment, and exposure and development

Prof. Jin-Heong Yim

Photoresist

Photoresist is an organic polymer which changes its chemical structure when exposed to ultraviolet light.

It contains a light-sensitive substance whose properties allow image transfer onto a printed circuit board.

There are two types of photoresist: positive and negative

Prof. Jin-Heong Yim

Substrate

PhotoresistCoatingCoating

Mask

hu

EtchEtch

StripStrip

Negative PositiveExposureExposure

DevelopDevelop

PR Process

Prof. Jin-Heong Yim

1. More sensitive photoresists (Very high efficiency)

2. Brighter light source (e.g. KrF laser for 248nm and ArF laser for 193nm)

Solution :

"Catalytic Cycle"

Photoacid Generator(PAG)

CH

3

O

CH2

CO C

CH

CH

CH3

3O

n

hv

H +

H +

OH

CH CH2

n

+ CO2 +H3C

CCH3

+ CH2

H

H3CC

CH3

CH2+t-BOC

Strong Acid

Chemical Amplification

Prof. Jin-Heong Yim

UV

Coat

Expose

Development

Si

III

II

II

II

II I

Si

III

II

II IP

PPP

PP

Si

II

III

I II

OH

CH3

CH2

n

Novolac Resin (N)

ON2

R R

COOH

(Base insoluble) (Base soluble)DNQ(Diazonaphthoquinone)

Inhibitor (I)Carboxylic acidPhotoproduct (P)

UV

Develop.Rate

novolac+ photoproducts

novolac+ DNQ

pure novolac

Reaction Coordinate

How Novolac/DNQ Resists Work

Prof. Jin-Heong Yim

Two Types of Photoresist

Exposure to UV light makes it more soluble in the developer

Exposed resist is washed away by developer so that the unexposed substrate remains

Results in an exact copy of the original design

Exposure to UV light causes the resist to polymerize, and thus be more difficult to dissolve

Developer removes the unexposed resist

This is like a photographic negative of the pattern

Positive Photoresist Negative Photoresist

Prof. Jin-Heong Yim

Preparation and Priming

Prepare the substrate (silicon wafer):Wash with appropriate solvent to remove any dirt and other impurities

Acetone, MeOH, TCEDry in Oven at 150°C for 10 min.Place on hotplate and cover with petri dish, let temp. stabilize at 115°C.

Deposit Primer (optional)Chemical that coats the substrate and allows for better adhesion of the resist

Prof. Jin-Heong Yim

Spin Coating

Spin-coat the photoresist onto the surface of the wafer

RPM: 1000-7000Time: ~30 secProduces a thin uniform layer of photoresiston the wafer surface.

Use red/amber safe light at this stage

Prof. Jin-Heong Yim

Soft Baking

Put on hotplate, or in oven

Temperature: 65°C-115°C, Time: 1-5 min

Removes volatile solvents from the coating

Makes photoresist imageable

Hardens to amorphous solid

Be careful not to overbake and destroy the sensitizer

Prof. Jin-Heong Yim

Mask Alignment and Exposure

Photomask is a square glass plate with a patterned emulsion of metal film on one sideAfter alignment, the photoresist is exposed to UV lightThree primary exposure methods: contact, proximity, and projection

Prof. Jin-Heong Yim

Exposure Methods

Prof. Jin-Heong Yim

* Use of shorter wavelength for exposure to get smaller feature size

“Lens Equation”

R= k1λNA

R: Resolution (feature size)λ:wavelength of light sourcek1:Process factorNA: Numerical aperture

Light

CoatedWafer

θ

(NA= sin θ)

Lens

Resolution of Lithographic Process

Prof. Jin-Heong Yim

Photoresist DeveloperHighly pure buffered alkaline solution

Removes proper layer of photoresist upon contact or immersion

Degree of exposure affects the resolution curves of the resist

Prof. Jin-Heong Yim

Hard Baking

Final step in the photolithographic processNot always necessary; depends on the resist

Hardens the photoresistImproves adhesion of the photoresist to the wafer surface