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A.G. Andreou 2000 1 520/580.495 Photolithography (II) Andreas G. Andreou Lecture notes from Positive Photoresists and Photolithography by R. Darling http://www.engr.washington.edu/~cam/PROCESSES

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A.G. Andreou 2000 1

520/580.495Photolithography (II)

Andreas G. Andreou

Lecture notes from

Positive Photoresists and Photolithography by R. Darling

http://www.engr.washington.edu/~cam/PROCESSES

A.G. Andreou 2000 2

Lecture Summary

� Positive Photoresists� Photoresist Spinning� Mask Aligning and Exposure

R. B. Darling / EE-527

Advantages of Positive Photoresists

• Most commonly used in the IC industry.• Superior to negative photoresists because:

– They do not swell during development.– They are capable of finer resolution.– They are reasonably resistant to plasma processing operations.

R. B. Darling / EE-527

Phenolic Resins - 1

• Phenolic resins are condensation polymers of aromaticalcohols and formaldehyde.

• Bakelite was the first thermosetting plastic.• Phenolic resins are readily cross-linked by thermal

activation into rigid forms.• Most phenolic resins are readily dissolved by aqueous

alkaline solutions, e.g. NaOH, KOH, NH4OH.

R. B. Darling / EE-527

Phenolic Resins - 2OH

HC

H

O

phenol

formaldehyde

OH OH OHH2C

H2C

H2C

bakelite

HO

Hwater

+

+

R. B. Darling / EE-527

Phenolic Resins - 3OH

HC

H

O

formaldehyde

OH OH OHH2C

H2C

H2C

HO

Hwater

+

+

CH3

CH3 CH3 CH3

para-cresol

novolac

R. B. Darling / EE-527

Important Properties of the Base Phenolic Resin

• average molecular weight– typically in the range of 1000 to 3000 g/mole– (8 to 25 repeating units in the polymer chain)

• dispersity of the molecular weights• isomeric composition of the cresols

– ortho-cresol– meta-cresol– para-cresol

• relative position of the methylene linkages (--CH2--)

R. B. Darling / EE-527

Cresol IsomersOH

OHH2C

CH3

CH3

para-cresol

OHOH

CH3

CH3

ortho-cresol meta-cresol

OH

CH3

OH

CH3

H2C

H2C

ortho-cresol resin meta-cresol resin para-cresol resin

R. B. Darling / EE-527

Cresol Isomer Properties

• Single isomers and smaller molecular weights are desirable• Manufacture of positive photoresist relies heavily upon

obtaining only a single isomer of the resin, usually para-cresol.

• Each monomer is [C8H8O] (120.151 g/mole)

Isomer Methylene Link Molecular Weight Dissolution Rate Plastic Flow Temp.

ortho-cresol 3 2100 g/mole 2.7 A/sec 85 C

meta-cresol 1 15000 g/mole 0.7 A/sec 73 C

para-cresol 1 1600 g/mole 3.0 A/sec 119 C

R. B. Darling / EE-527

Photoreaction in a Positive Photoresist

O

N2

COOH

+

+

H2O

N2

diazonaphthaquinone (DQ)

indene carboxylic acid (ICA)

R. B. Darling / EE-527

Dissolution of Phenolic Resins - 1

• Because of the OH groups, phenolic resins are hydrophylicand are readily dissolved by aqueous alkaline solutions.

• Diazonaphthaquinone (DQ) is a hydrophobic and non-ionizable compound.

• When phenolic resins are impregnated with DQ, theybecome hydrophobic and their dissolution is greatlyinhibited.

• After exposure, DQ is converted into indene carboxylicacid (ICA) which is hydrophylic and very ionizable.– This allows the developer to wet and penetrate the novolac resin.

• Phenolic resins which contain ICA instead of DQ arereadily dissolved by aqueous alkaline developers.

R. B. Darling / EE-527

Dissolution of Phenolic Resins - 2

0 10 20 30 40

0.1

1.0

10

100

1000

exposed novolac + DQ

unexposed novolac + DQ

DQ concentration, weight percent

Dis

solu

tion

rate

, nm

/sec

in 0

.15

M N

aOH

R. B. Darling / EE-527

DQ Primary PhotoreactionO

N2

O

N2+

+ H2O

CO COOH

diazonaphthaquinone (DQ) a carbene

a ketene indene carboxylic acid (ICA)

ring contraction

hydration

photolysis

See Otto Suess, 1944 and 1947 papers in Annalen.

R. B. Darling / EE-527

Physical Requirements on the Photoactive Component

• Need an overlap of the absorption spectrum with theemission spectrum of the exposure source, e.g. a Hg lamp.

• Need bleachability at the exposure wavelength so that thephotoreaction is able to reach the resist-substrate interface.

• Need compatibility with the base resin (novolac) so thatthe two form a single, miscible phase.

• Need thermal stability so that the photoactive dissolutioninhibitor does not break down at prebake temperatures.

• Photoactive dissolution inhibitors are often modified toalter their spectral absorption, thermal stability, andmiscibility characteristics.

R. B. Darling / EE-527

Spectral Absorption of Novolac, DQ, and ICA

200 300 400 500 600

1.0

10

100

1000

10,000

Wavelength, λ, nm

313

365

405

437

Hg arc lamp linesnovolac

DQ (unexposed photoinhibitor)

ICA (exposed photoinhibitor)

R. B. Darling / EE-527

Primary Components of a Positive Photoresist

• Non-photosensitive base phenolic resin– usually novolac

• Photosensitive dissolution inhibitor– usually a DQ-derived compound

• Coating solvent– n-butyl acetate– xylene– 2-ethoxyethyl acetate

• very carcinogenic, TLV = 5 ppm• now removed from most positive photoresists

R. B. Darling / EE-527

Secondary Components of a Positive Photoresist

• Antioxidants• Radical scavengers• Amines to absorb O2 and ketenes• Wetting agents• Dyes to alter the spectral absorption characteristics• Adhesion promoters• Coating aids

R. B. Darling / EE-527

Sensitometric Curve for a Positive Photoresist

1 10 100 1000 10,000

0.0

0.5

1.0

Exposure Dose, D, mJ/cm2

Developed Resist Thicknessnormalized to 1.0

working pointfor the resist

R. B. Darling / EE-527

Novolac Dissolution - 1

• A minimum concentration of [OH-] is required to producea net forward rate:

H3C OH

CH2

+ OH-

H2O+H3C O-

CH2

The dissolution rate is R = kCn, where C is the base concentration.

For NaOH solutions, R = (1.3 x 105) [Na+]1 [OH-]3.7 Angstroms/second.

R. B. Darling / EE-527

Novolac Dissolution - 2

Dissolution Rate, Angstroms/secondSolutionUnexposed Exposed

0.15 M NaOH 20 14000.15 M KOH 10 860

0.15 M NaOH +0.1 M Na2SiO3

270 3400

0.15 M NaOH +0.1 M Na3PO4

350 2800

0.15 M NaOH +0.1 M Na2CO3

270 2400

Typical data for different developer solutions:

R. B. Darling / EE-527

Positive Photoresist Exposure Latitude

-0.4

0.0

+0.4

Exposure Dose, D, mJ/cm2

-0.2

+0.2

Critical Dimension Shift, µm

200180160 220150 170 190 210 230

Exposure Latitude

BLOAT

SHRINK

lines and islands

spaces and windows

Kodak Micro Positive 820

30 min. @ 95 C prebake in convection oven

30 sec. @ 71 C develop with agitation, 1:2 mix

working point: 185 mJ/cm2

R. B. Darling / EE-527

Positive Photoresist Prebake Latitude

-0.4

0.0

+0.4

Prebake Temperature, degrees C

-0.2

+0.2

Critical Dimension Shift, µm

Prebake Latitude

BLOAT

SHRINK

lines and islands

spaces and windows

Kodak Micro Positive 820

185 mJ/cm2 exposure

30 sec. @ 71 C develop with agitation, 1:2 mix

working point: 95 C

90 95858075 100 105 110 115

R. B. Darling / EE-527

Photoresist Spin Coating

• Wafer is held on a spinner chuck by vacuum and resist iscoated to uniform thickness by spin coating.

• Typically 3000-6000 rpm for 15-30 seconds.• Resist thickness is set by:

– primarily resist viscosity– secondarily spinner rotational speed

• Resist thickness is given by t = kp2/w1/2, where– k = spinner constant, typically 80-100– p = resist solids content in percent– w = spinner rotational speed in rpm/1000

• Most resist thicknesses are 1-2 µm for commercial Siprocesses.

R. B. Darling / EE-527

Photoresist Spin Coating

vacuum chuck

resist dispenser

wafer to be coated

excess resist flies off during rotation

time

speed

slowcoat

level out

spindown

photoresist

R. B. Darling / EE-527

Spinning Artifacts

• Striations– ~ 30 nm variations in resist thickness due to nonuniform drying of

solvent during spin coating– ~ 80-100 µm periodicity, radially out from center of wafer

• Edge Bead– residual ridge in resist at edge of wafer– can be up to 20-30 times the nominal thickness of the resist– radius on wafer edge greatly reduces the edge bead height– non-circular wafers greatly increase the edge bead height– edge bead removers are solvents that are spun on after resist

coating and which partially dissolve away the edge bead

• Streaks– radial patterns caused by hard particles whose diameter are greater

than the resist thickness

R. B. Darling / EE-527

Prebake (Soft Bake) - 1

• Used to evaporate the coating solvent and to densify theresist after spin coating.

• Typical thermal cycles:– 90-100°C for 20 min. in a convection oven– 75-85°C for 45 sec. on a hot plate

• Commercially, microwave heating or IR lamps are alsoused in production lines.

• Hot plating the resist is usually faster, more controllable,and does not trap solvent like convection oven baking.

wafer

photoresist

hot plate chuck

coating solventis driven off

R. B. Darling / EE-527

Prebake (Soft Bake) - 2

• A narrow time-temperature window is needed to achieveproper linewidth control.

• The thickness of the resist is usually decreased by 25 %during prebake for both positive and negative resists.

• Less prebake increases the development rate:

50 60 70 80 90 100

0

100

200

300

400

temperature, °C

dissolution rate, nm/sec.

R. B. Darling / EE-527

Prebake (Soft Bake) - 3

• Convection ovens:– Solvent at surface of resist is evaporated first, which can cause

resist to develop impermeable skin, trapping the remaining solventinside

– Heating must go slow to avoid solvent burst effects

• Conduction (hot plate):– Need an extremely smooth surface for good thermal contact and

heating uniformity– Temperature rise starts at bottom of wafer and works upward,

more thoroughly evaporating the coating solvent– Generally much faster and more suitable for automation

R. B. Darling / EE-527

Overview of Align/Expose/Develop Steps

uniform UV exposure illumination

chrome on glass photomask

photoresist (PR)

substrate waferlatent image created inphotoresist after exposure

(x,y,θ) alignment ofmask to substrate

wet chemical development

NEGATIVE PHOTORESIST POSITIVE PHOTORESISTPhotoresist is photopolymerized whereexposed and rendered insoluble to thedeveloper solution.

Exposure decomposes a developmentinhibitor and developer solution onlydissolves photoresist in the exposed areas.

R. B. Darling / EE-527

Photomasks

• Master patterns which are transferred to wafers• Types:

– photographic emulsion on soda lime glass (cheapest)– Fe2O3 on soda lime glass– Cr on soda lime glass– Cr on quartz glass (most expensive, needed for deep UV litho)

• Dimensions:– 4” x 4” x 0.060” for 3-inch wafers– 5” x 5” x 0.060” for 4-inch wafers

• Polarity:– “light-field” = mostly clear, drawn feature = opaque– “dark-field” = mostly opaque, drawn feature = clear

R. B. Darling / EE-527

Mask to Wafer Alignment - 1

– 3 degrees of freedom between mask and wafer: (x,y,θ)– Use alignment marks on mask and wafer to register patterns prior

to exposure.– Modern process lines (steppers) use automatic pattern recognition

and alignment systems.• Usually takes 1-5 seconds to align and expose on a modern stepper.• Human operators usually take 30-45 seconds with well-designed

alignment marks.

alignment mark on wafer,created from prior processing step

alignment mark on mask,open window in chrome throughwhich mark on wafer can be seen

R. B. Darling / EE-527

Mask to Wafer Alignment - 2

• Normally requires at least two alignment mark sets onopposite sides of wafer or stepped region.

• Use a split-field microscope to make alignment easier:

L R

L R

R. B. Darling / EE-527

Mask to Wafer Alignment - 3

• Visual alignment:– Process of getting wafer coarsely centered under mask– All that is needed for the first mask of the set, since no patterns on

the wafer exist yet– Accomplished by special windows on a dark field mask

R. B. Darling / EE-527

Oriel Alignment Fixture

vacuum chuckmask holder

wafer stage1 2 3

mask

mask retainer wafer or substrate

height adjustment ring

(x,y,θ) adjustment micrometers

foam rubbermask gasket

main chambergasket

R. B. Darling / EE-527

Postbake (Hard Bake) - 1

• Used to stabilize and harden the developed photoresistprior to processing steps that the resist will mask.

• Main parameter is the plastic flow or glass transitiontemperature.

• Postbake removes any remaining traces of the coatingsolvent or developer.

• This eliminates the solvent burst effects in vacuumprocessing.

• Postbake introduces some stress into the photoresist.• Some shrinkage of the photoresist may occur.• Longer or hotter postbake makes resist removal much

more difficult.

R. B. Darling / EE-527

Postbake (Hard Bake) - 2

• Firm postbake is needed for acid etching, e.g. BOE.• Postbake is not needed for processes in which a soft resist

is desired, e.g. metal liftoff patterning.• Photoresist will undergo plastic flow with sufficient time

and/or temperature:– Resist reflow can be used for tailoring sidewall angles.

100°C 110°C 120°C 130°C 140°C

R. B. Darling / EE-527

Photoresist Removal (Stripping)

• Want to remove the photoresist and any of its residues.• Simple solvents are generally sufficient for non-postbaked

photoresists:– Positive photoresists:

• acetone• trichloroethylene (TCE)• phenol-based strippers (Indus-Ri-Chem J-100)

– Negative photoresists:• methyl ethyl ketone (MEK), CH3COC2H5

• methyl isobutyl ketone (MIBK), CH3COC4H9

• Plasma etching with O2 (ashing) is also effective forremoving organic polymer debris.– Also: Shipley 1165 stripper (contains n-methyl-2-pyrrolidone),

which is effective on hard, postbaked resist.