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Phase shift mask technology Light (an electromagnetic wave) has both

amplitude and phase A conventional photomask consists of a quartzplate with a patterned opaque layer

Constructive interference between openings

enhance both the electric field and intensity,reducing both contrast and resolution By shifting the phase in adjacent openings by

180, destructive interference can minimizeunwanted intensity between openings

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Phase-shifted mask technology

electricfield at

mask

electricfield atwafer

intensityat wafer

mask

phaseshifter

conventional mask phase shift mask

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Standing wave effects

The projection of a perfectimage onto a resistsurface will notnecessarily result in the

replication of that image! With monochromatic light,standing waves create aperiodic intensitydistribution perpendicularto the plane of the resist.nding waves cause steps to occur at the edge of an expotern; minimized by baking the photoresist so that it will fl

using an antireflection coating at the wafer surface

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Reflections off topographic features

erference between the imaging beam and its reflection off rface of the wafer and topographic features is a major causewidth variationsriations can be minimized by planarizing the wafer either bhing or by chemical-mechanical polishingtireflective polymers underneath the resist also helpth planarization and a.r. coatings help, but both increase th

mplexity of the process.

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A 1983 prediction...

After consideration of all factors which limitresolution such as exposure hardware, resistsystems, registration, alignment, and linewidthcontrol, there is a general consensus that theuseful resolution limit of photolithography maylie somewhere between 0.4 and 0.8 m anddepends on such factors as the implemen-tation

of short wavelength UV and the ability toaccurately place images from a projection toolonto a silicon wafer.

L.E. Thomas and M.J. Bowden, in Introduction to

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Photoresists (1)

general properties of polymers

photoresist types photoresist characterization

(dissolution behavior,sensitivity, contrast,resolution, etchingresistance)

resist materials general characteristics negative resists positive resists

photoresist processing

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Generic properties of polymers

Polymers are long chain, usually organic (C,O, H) molecules with high molecular weights(1000 >1,000,000)

Thermoplastic polymers -- chains are free to

move past each other at high temperatures,and become entangled at low temperatures

linear chains (LDPE) branched chains(HDPE)

Thermoset polymers -- chains arecrosslinked together to form a three-dimensional network (example: epoxy resins)

The properties of a polymer are determined

by its chemical constituents, its molecularwei ht (and M.W. distribution) and the de ree

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Important polymers in photoresists

OH

CH 3C O

H

H

m -cresol formaldehyde

OH

CH 3

CH 2

OH

CH 3CH 2

OH

CH 3

CH 2

CH 2 CH 2

novolacpolymer

(novolac = new lacque

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Important polymers in photoresists

C C C CH

H

CH 3H H

H

C C C CH

H

CH 3H H

Hcis -isoprene poly ( cis -isoprene)

C C

H

H

CH 3

CH 3

O

C O

C C

H

H

CH 3

CH 3

O

C O

methyl methacrylate poly(methyl methacrylate) (PMMA)

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Mechanical properties of polymers

The mechanical properties of polymers (whichare important to photoresist processing)strongly depend on the long chain nature of these materials and both intra- and inter-chain

reactions The glass transition temperature T g is an

important parameter T < T g -- only short-range motions of the

polymer chain are possible elastic behavior

T > T g -- long-range motions of the polymerchain are possible viscous behavior

Both the chemical structure (crosslinking

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Positive versus negative resists

development

positive-tone resist negative-tone resist

photoresist

substrate

mask

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Positive versus negative photoresists

Positive photoresist -- exposure to lightincreases the solubility of the polymer

(opening in mask opening in resist)

Negative photoresist -- exposure to lightdecreases the solubility of the polymer

(opening in mask closed area in resist)

Both positive and negative resists are used,depending on device design, process flow, and

the demands of the lithographic process(examples and reasons will be given)

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Photoresist characterization

There are a few common characteristics of allphotoresists that are used to gauge theirperformance

dissolution behavior sensitivity contrast resolution etching resistance

The choice of a particular photolithographyprocess (optical system + photoresist system)will depend on compromises between these(and other!) factors

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Photoresist dissolution behavior

Photoresist dissolution must be determined foreach combination of exposure system,photoresist, and development system

Dissolution curves give an operational definition

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Photoresist sensitivity -- contrast curves

1.01.0

0.50.5

N o r m a

l i z e

d t h i c k n e s s a

f t e r

d e v e

l o p m e n

t

N o r m a

l i z e

d

t h i c k n e s s a

f t e r

d e v e

l o p m e n

t

log Dose log Dose

Do

Do

D 100

D 100

positive resist negative resist

Do = incipient dose : minimum dose required for agiven process to emerge (note -- varies with processand development conditions)

D100 = completion dose : dose at which the resist is

completely dissolved positive resists) or at which theresist is com letel inert (ne ative resist)

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The contrast is defined as

Contrast is the ability of a photoresist todistinguish between light and dark portions of the mask

In general, the higher the contrast, thesharper are the edge profiles of developedlines ( highly desirable! )

Contrast can be varied by adjustment of theresist processing parameters

Photoresist sensitivity -- contrast curves

0

10010log

1

D

D=

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Resolution is the ability of a resist to resolvefine lines in the final printed pattern

Resolution depends strongly on thechemistry of the resist and the developer

system Other factors can affect resolution

Exposure hardware (Chapter 7)Resist deformation from thermal flow

during processing If a resist is heated above its glass transition

during processing, thermal flow may distortthe exposed pattern

plasma etching

Photoresist resolution

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After the photoresist is exposed, it isexposed to corrosive or physically-abusiveenvironments in subsequent processing Wet etches strong acids or bases

Dry etches oxygen plasma ashesorganic materials; chlorinated (for metals)and fluorinated (for silicon) plasmas arevery corrosive to photoresists

Etching resistance is the ability of a resistto withstand the conditions necessary totransfer the pattern Etching resistance is a strong function of

resist chemistry

Photoresist etching resistance

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A polymer-based photoresist mustmeet rigorous requirements: high sensitivity high contrast

high T g good etch resistance good resolution easy processing long shelf life minimal solvent use reasonable cost

Resist materials -- overview

not your basicTupperware!

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Negative resists were the first to be used insemiconductor device fabrication Polymer matrix + organic sensitizer

matrix: poly( cis -isoprene) -- a synthetic

rubber sensitizer: bis(aryl)azide

Resist materials -- negative resists

2,6-bis(4-azidobenzylidencyclohexanone

hanism -- the sensitizer loses nitrogen under h and generhly reactive nitrene; this reacts with the polymer to crossli

lowers its solubility by organic solvents (like vulcanization

O

N 3N 3

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Resolution in negative resists is limited byswelling in the exposed crosslinked areascaused by solvent uptake:

Problems with negative resists

during development after developmentsolvent

exposed PR unexposed PR exposed PR swollen by solvent

Line width of negative resists limited to ~3 m Organic solvents a problem ( e nvironment-s afety- h ealth)

Advantages: greater process latitude, much

lower cost - -

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Positive resists have gained popularitybecause of their superior resolution, betteretching resistance, and thermal stability

Polymer matrix + organic sensitizer

matrix: novolac resin (cresol/formaldehydepolymer) sensitizer: diazoquinone (DQ)

Resist materials -- positive resists

hanism -- upon photolysis, the DQ sensitizer loses nitrogenverted into a carboxylic acid; the irradiated areas of the rbe dissolved in a strong base (KOH, NaOH)

SO 2 R

N 2

O

the specific organic group R pla ysonly a secondary role in the resist exposure process

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Specific reaction mechanism of DQ

SO 2 R

N 2

O

SO 2 R

O

SO 2 R

O

C

SO 2 R

O

C OH

h , -N 2

H 2 O

(1) (2)

(3)(4)

DQ carbene

keteneindenecarboxylicacid (ICA)

Wolff rearrangement

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Photoresist processingWafer preparation

Adhesion promoter

Resist application

SoftbakeExposure

Post-exposure bake

Develop

Hardbake

Post-development treatments

This will berepeated 15 to

30 times duringthe processing

of a given wafer

-- all lithography steps must be

aligned to eachother

These steps arebeing

performed onhighly value-

added wafers --any error will

negate (trash)all of the prior

work