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OPTICAL EXPOSURES BY AJAL.A.J When employees are trained to work safely they should be able to anticipate and avoid injury from job-related hazards.

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When employees are trained to work safely they should be able to anticipate and avoid injury from job-related hazards. • Surface Preparation • Coating (Spin Casting) • Pre-Bake (Soft Bake) • Alignment • Exposure • Development • Post-Bake (Hard Bake) • Processing Using the Photoresist as a Masking Film • Stripping • Post Processing Cleaning (Ashing)

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Page 1: Optical exposures

OPTICAL EXPOSURES

BY AJAL.A.J

When employees are trained to work safely they should be able to anticipate and avoid injury from

job-related hazards.

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Eye and Face Protection

Thousands of people are blinded each year from work-related eye injuries. According to the Bureau of Labor Statistics (BLS), nearly three out of five workers are injured while failing to wear eye and face protection.

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EYEWEAR LABELS

All eyewear must be labeled with wavelength and optical density.

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CDRH CLASS WARNING LABELS

CLASS II LASER PRODUCT

Laser RadiationDo Not Stare Into Beam

Helium Neon Laser1 milliwatt max/cw

CLASS IV Laser Product

VISIBLE LASER RADIATION-AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION

Argon IonWavelength: 488/514 nmOutput Power 5 W

Class IIClass IIIa with expanded beam

Class IIIa with small beamClass IIIbClass IV

Laser-Professionals.com

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LASER PROTECTIVE BARRIER

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INTERNATIONAL LASERWARNING LABELS

Symbol and Border: BlackBackground: Yellow

Legend and Border: BlackBackground: Yellow

Laser-Professionals.com

INVISIBLE LASER RADIATIONAVOID EYE OR SKIN EXPOSURE

TO DIRECT OR SCATTERED RADIATIONCLASS 4 LASER PRODUCT

WAVELENGTH 10,600 nmMAX LASER POWER 200 W

EN60825-1 1998

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CLASS 4 LASER

ND:YAG 1064 nm100 Watts Max. Average Power

VISIBLE and/ or INVISIBLE LASER RADIATION-AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION.

Controlled Area Warning SignLaser-Professionals.com

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SKIN BURN FROM CO2 LASER EXPOSURE

Accidental exposure to partial reflection of 2000 W CO2 laser beamfrom metal surface during cutting

Laser-Professionals.com

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HUMAN EYE

Choroid

Aqueous

Cornea

Macula

Optic Nerve

Sclera

Vitreous

RetinaLens

Laser-Professionals.com

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25

Photo courtesy of U S Air Force

THERMAL BURNSON

PRIMATE RETINA

Laser-Professionals.com

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MULTIPLE PULSE RETINAL INJURY

Laser-Professionals.com

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• Most beam injuries occur during alignment.

• Only trained personnel may align (NO EXCEPTIONS!)

• safety eyewear is required for class 3B and class 4

beam alignment.

• ANSI REQUIRES approved, written alignment

procedures for ALL class 4 laser alignment activities

and recommends them for class 3B.

SAFE BEAM ALIGNMENT

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Photo courtesy of U S Army Center for Health Promotion and Preventive Medicine

EYE INJURY BY Q-SWITCHED LASERRetinal Injury produced by four pulses from a Nd:YAG laser range finder.

Laser-Professionals.com

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

• Photolithography: an optical means for transferring patterns onto a substrate.

• Patterns are first transferred to an imagable photoresist layer.

• Photoresist is a liquid film that is spread out onto a substrate, exposed with a desired pattern, and developed into a selectively placed layer for subsequent processing.

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

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Overview of the Photolithography Process

1. • Surface Preparation2. • Coating (Spin Casting)3. • Pre-Bake (Soft Bake)4. • Alignment5. • Exposure6. • Development7. • Post-Bake (Hard Bake)8. • Processing Using the Photoresist as

a Masking Film9. • Stripping10. • Post Processing Cleaning (Ashing)

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1] Surface PreparationWafer Cleaning• Typical contaminants that must be removed prior to photoresist coating:

• dust from scribing or cleaving (minimized by laser scribing)

• atmospheric dust (minimized by good clean room practice)

• abrasive particles (from lapping or CMP)

• lint from wipers (minimized by using lint-free wipers)

• photoresist residue from previous photolithography (minimized by performing oxygen plasma ashing)

• bacteria (minimized by good DI water system)

• films from other sources:

– solvent residue

– H 2O residue

– photoresist or developer residue

– oil

– silicone

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Wafer Priming• Adhesion promoters are used to assist resist coating.

• Resist adhesion factors:

• moisture content on surface

• wetting characteristics of resist

• type of primer

• delay in exposure and prebake

• resist chemistry

• surface smoothness

• stress from coating process

• surface contamination

• Ideally want no H 2O on wafer surface

– Wafers are given a “singe” step prior to priming and coating 15 minutes in 80-90°C convection oven

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2] Coating (Spin Casting)

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Photoresist Spin Coating• Wafer is held on a spinner chuck by vacuum and resist is coated 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 = kp 2 /w 1/2 , 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 mm for commercial Si processes.

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Spin Coater

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3] Pre-Bake (Soft Bake)

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Prebake (Soft Bake) • Used to evaporate the coating solvent and to densify the resist after spin coating.

• Typical thermal cycles:

– 90-10°C for 20 min. in a convection oven

– 75-8°C for 45 sec. on a hot plate

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

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

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Mask Aligner

4] Alignment

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Mask to Wafer Alignment - 1– 3 degrees of freedom between mask and wafer: (x,y,q)

– 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.

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Mask to Wafer Alignment - 2• Normally requires at least two alignment mark sets on

opposite sides of wafer or stepped region.

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

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Some more Alignment Marks

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Optical Exposure• Projection Optics

• Numerical Aperture

• Raleigh Criterion

• Coherence

• Optical Correction & Phase Shift

•Selection

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4] Alignment

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5] Exposure

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6] Development

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7] Post-Bake (Hard Bake)

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8 ] Processing Using the Photoresist as a Masking Film

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9 ] Stripping

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10 ] Post Processing Cleaning (Ashing)

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STEPPER

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The Head of a Stepper

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Projection Lithography Requirements

– b = minimum feature size (spot or line)

– 2b = minimum period of line-space pattern

– l = exposure wavelength

– Using b = f qmin , obtain that b » l/2NA.

– The depth of focus can be shown to be d f = ± l/2(NA)2

– A “voxel” is a volume pixel.

– For highest resolution lithograpy, desire the tallest aspect ratio

voxel.

– Thus, wish to maximize the ratio d f /b = 1/NA.

– SO: it all depends upon the NA of the lens!

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• Short exposure wavelengths can create standing waves in a layer of photoresist. Regions of constructive interference create increased exposure.

• These can impair the structure of the resist, but can be eliminated by:

– use of multiple wavelength sources

– postbaking

• Effects are most noticeable at the edge of the resist.

• Standing waves are enhanced by reflective wafer surfaces.

• If the wafer or substrate is transparent, reflections from the aligner chuck can create standing wave patterns, also.

– This can be eliminated by using:

• a flat black chuck (anodized aluminum)

• an optical absorber under the wafer (lint free black paper)

• a transparent glass chuck (used on Karl Suss MJB3)

• Exposures can be greatly miscalculated by the presence of standing waves and reflective wafers or chucks.

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WHAT IS A PHOTOMASK?

Photomasks are high precision plates containing microscopic images of

electronic circuits. Photomasks are made from very flat pieces of quartz or glass with a

layer of chrome on one side. Etched in the chrome is a portion of an electronic circuit

design. This circuit design on the mask is also called geometry.

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MATERIAL USED TO MAKE PHOTOMASKS:

There are four types of material used to make photomasks; quartz (the most commonly used and most expensive), LE, soda lime, and white crown. The mask sizes can range from 3 inches square to 7 inches square and 7.25 inches round.

The thickness of the masks ranges from 60 mils to 250 mils. Currently the most common sizes of masks used are 5 inches square 90 mils thick and 6 inches square 250 mils thick.

The quartz or glass (or substrate) has a layer of chrome on one side. The chrome is covered with an AR (anti-reflective) coating and a photosensitive resist.

The substrate with chrome, AR, and resist is known as a blank.

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Standard Elements of PhotoMask

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Defects in Photomask

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Postbake (Hard Bake) - 1• Used to stabilize and harden the developed photoresist prior to processing steps that the resist will mask.

• Main parameter is the plastic flow or glass transition temperature.

• Postbake removes any remaining traces of the coating solvent or developer.

• This eliminates the solvent burst effects in vacuum processing.

• Postbake introduces some stress into the photoresist.

• Some shrinkage of the photoresist may occur.

• Longer or hotter postbake makes resist removal much more difficult.

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Postbake (Hard Bake)• 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.

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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), CH 3 COC 2 H 5

• methyl isobutyl ketone (MIBK), CH 3 COC 4 H 9

• Plasma etching with O 2 (ashing) is also effective for removing organic polymer debris.

– Also: Shipley 1165 stripper (contains n-methyl-2-pyrrolidone), which is effective on hard, postbaked resist.

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PhotoResist

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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.

Positive PhotoResist

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Requirements: the Photoactive Component• Need an overlap of the absorption spectrum with the emission spectrum of the exposure source, e.g. a Hg lamp.

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

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

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

• Photoactive dissolution inhibitors are often modified to alter their spectral absorption, thermal stability, and miscibility characteristics.

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Bleaching of a Positive Photoresist– The solution to the coupled Dill equations predicts a sharp boundary between exposed and unexposed regions of the resist.

The boundary is the front of a bleaching edge which propagates downward to the substrate as the resist is exposed. This makes the wall angle more dependent upon the {A,B,C} Dill parameters than upon the exposure wavelength, and gives positive photoresists very high resolution.

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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

Secondary Components of a Positive Photoresist

• Antioxidants

• Radical scavengers

• Amines to absorb O 2 and ketenes

• Wetting agents

• Dyes to alter the spectral absorption characteristics

• Adhesion promoters

• Coating aids

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Negative Photoresist Ingredients

• 1. Non-photosensitive substrate material

• 2. Photosensitive cross-linking agent

• 3. Coating solvent

• 4. Other additives: (usually proprietary)

– antioxidants

– radical scavengers

– amines; to absorb O2 during exposure

– wetting agents

– adhesion promoters

– coating aids

– dyes

Negative PhotoResist

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Negative Photoresist Development - 1• The unexposed (uncross-linked) areas of resist as well as polymer chains that have not been cross-linked to the overall network of the gel must be dissolved during development.

• Negative photoresist developers are solvents which swell the resist, allowing uncross-linked polymer chains to untangle and be washed away.

• A sequence of solvents is often used to keep the swelling reversible.

• The swelling of the resist during development is the largest contributor to loss of features and linewidth limitations.

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The Gel Point– All sites for cross-linking (chromophores) are equally likely; thus, larger polymer chains are more likely to bind together than small ones.

– A many-branched supermolecule results from increased exposure.

– This supermolecule permeates the irradiated area forming a lattice which solvent atoms can penetrate, but not disperse.

– The polymer chains have at this point been rendered insoluble to the solvent, and the exposure required to produce this is called the Gel Point.

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