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Lithography Fabrication

Jadhav Avinash J 2K13E11 Savitri Bai Phule Pune University

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What is Lithography?

Lithography(Greek word) means printing is done on stone.

Photo-litho-graphy: light-silicon wafer-printing.

Components in photolithography: (1)Mask (2)Photoresist (3)UV exposure

system

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Overview of the Photolithography Process

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Steps Used in Photolithography

Surface cleaning Spin coating with photoresist Soft baking Mask alignment Exposure Development Hard bakingPlasma Etch-Or Add Layer Post process cleaningFinal Inspection

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Surface Cleaning

Typical contaminants that must be removed prior to photoresist coating: dust from scribing or cleaving (minimized by laser scribing)

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

atmospheric dust (minimized by good clean room practice)

bacteria (minimized by good DI water system)

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films from other sources:-solvent residue-H2 O residue-photoresist or developer residue-silicone

For particularly troublesome grease, oil, or wax stains: Start with 2-5 min. soak in 1,1,1-trichloroethane (TCA) or trichloroethylene (TCE) with ultrasonic agitation prior to acetone

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Spin coating with photoresist

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

Most resist thicknesses are 1-2 μm for commercial Si processes.

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Resist thickness is given by t =square of( kp)/root of(w1), where,

k = spinner constant, typically 80-100p = resist solids content in percentw = spinner rotational speed in

rpm/1000

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Stages of Resist Coating

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Soft baking

Used to evaporate the coating solvent.

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

Microwave heating or IR lamps are also used

Optimizes light absorbance characteristics of photoresist

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Mask alignment & Exposure

Transfers the mask image to the resist-coated wafer

Activates photo-sensitive components of photoresist

Three types of masking(1) Contact printing (2) Proximity printing(3) Projection printing

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Contact printing capable of high resolution but has unacceptable defect densities. May be used in Development but not manufacturing.

Proximity printing cannot easily print features below a few mm in line width. Used in nano-technolgy.

Projection printing provides high resolution and low defect densities and dominates today.They print » 50 wafers/hour.

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Positive resist & Negative resist

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Development

Soluble areas of photoresist are dissolved by developer chemical

Visible patterns appear on wafer

Quality measures: –line resolution –uniformity –particles & defects

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Hard baking

Used to stabilize and harden the developed photoresist prior to processing steps that the resist will mask.

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

Higher temperature than soft bake (120-150 degree)

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Plasma Etch-Or Add Layer

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Post process cleaning

Plasma etching with O2 (ashing)Simple solvents are generally sufficient for

non- postbaked photoresists:Positive photoresists: aceton trichloroethylene (TCE)

Negative photoresists: methyl ethyl ketone methyl isobutyl ketone

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Final Inspection

Photoresist has been completely removed

Pattern on wafer matches mask pattern (positive resist)

Quality issues: –defects –particles –step height –critical dimensions

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Any Questions???