Concepts and principles of optical Xip lithography - · PDF fileConcepts and principles of optical lithography Francesc Pérez-Murano Institut de Microelectrònica de Barcelona (CNM-IMB,

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Institut de Microelectrònica de Barcelona

Escuela de verano de Jaca July 2011

Concepts and principles of optical

lithography

Francesc Pérez-Murano

Institut de Microelectrònica de Barcelona (CNM-IMB, CSIC)

[email protected]

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1 cm

10 cm

100 um

1 mm

1 um

10 um

10 nm

100 nm

1 nm

0,1 nm

mà

Gra de sorra

Diàmetre cabell humà

Bacteries

Molècula de DNA

Distància interatòmica

Oblia

Xip

Micromotors

Circuit integrat

Transistor MOS

Dispositius quàntics

Estructures atomiques

Microelectrònica

Nanotecnologia

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Nan

otec

hnol

ogy,

D. M

. Ten

nant

. A

IP/S

prin

ger,

New

Yor

k, 1

999

NanolithographiesNanolithographies

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Summary

Concept of optical lithography

Resists

Part ii

Associated processes

Part ii

Miniaturization

Limits optical lithography

Part i

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Escuela de verano de Jaca July 2011Bibliography

Nanoelectronics and Information. Chapter 9. Technology. Rainer Waser. Wiley-VCH 2003

Fundamentals of Microfabrication. Chapter 1. Marc Madou. CRC Press. 1997

Silicon Processing for the VLSI Era. Volume 1-Process Technology. Chapter 12 and 13. S. Wolf and R.N. Tauber. Lattice Press. 1986v

Principles of Lithography. Harry J. levinson. SPIE Press. 2004

Basic Books

Web sites http://www.intel.com/technology/silicon/index.htm http://www.microchemicals.de/products.html

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Summary


Resists

Part ii


Part ii

Miniaturization


Part i

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Summary


Resists

Part ii


Part ii

Miniaturization


Part i

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Fabrication using optical lithography

Mask

Resist

Exposition

Development

Etching

Initial substrate

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silicon substrate

oxide

photoresist

Positive Lithography

Island

Window

Areas exposed to light become photosoluble.

Resulting pattern after the resist is developed.

Shadow on photoresist

Exposed area of photoresist

Chrome island on glass mask

photoresist

silicon substrate

oxide

Ultraviolet Light

Positive Resist Tone

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Negative Lithography

Island

silicon substrate

oxide

photoresist

Window

Areas exposed to light become polymerized and sustain the develop chemical


photoresistoxide

silicon substrate

Ultraviolet Light




Negative Resist Tone

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Optical lithographyLithography: Image a mask on a wafer

Wavelength of the light sources:Near UV and deep UV

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Spectral distribution: Mercury Arc Lamp

i-line (365 nm)

g-line (435 nm)

h-line (405 nm)

Optical source for UV

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Light source

Optical system

MaskPhotoresistSample

Gap

Contact Proximity Projection

Exposition 1:1 1:1 5:1

Optical lithography modes

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UV optical lithography systems

Optical aligners Stepper

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Contact printing

R=MFS=(d·

d: Resist thicknessLight wavelenght

d= 1 m; = 435 nm; R=0.66 m

MFS: Minimum feature size

(difraction limited)

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Proximity printing

a h

R= MFS (Minimum feature size)

K: Experimental parameter (>1)

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Projection printing

MFS=R = k1λNA

NA: Numerical aperture

k1: technology constant(0.5 – 0.9)

k1= 0,66; = 435 nm; NA= 0.7; R = 0.4 m

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Resolution in projection printing

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Step-and scan system (stepper)

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1. Surface Preparation2. Photoresist Application3. Soft Bake4. Align & Expose*

5. Develop6. Hard Bake7. Inspection8. Etch9. Resist Strip10. Final Inspection

* Some processes may include a Post-exposure Bake

Ten Basic Steps of Photolithography

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1. Surface Preparation (HMDS vapor prime)

Dehydration bake in enclosed chamber with exhaust

Clean and dry wafer surface (hydrophobic)

Hexamethyldisilazane (HMDS)

Temp ~ 200 - 250C Time ~ 60 sec.

HMDS

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HEXAMETHYLSILIZANE (HDMS)Dehydration

Adhesion promotion by HDMS

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2. Photoresist Application

Wafer held onto vacuum chuck

Dispense ~5ml of photoresist

Slow spin ~ 500 rpm Ramp up to ~ 3000 -

5000 rpm Quality measures:

time speed thickness uniformity particles & defects vacuum chuck

spindleto vacuum

pump

photoresist dispenser

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Resist spinning thickness T depends on: Spin speed Solution concentration Molecular weight (measured by

intrinsic viscosity) In the equation for T, K is a

calibration constant, C the polymer concentration in grams per 100 ml solution, the intrinsic viscosity, and the number of rotations per minute (rpm)

Once the various exponential factors (, and ) have been determined the equation can be used to predict the thickness of the film that can be spun for various molecular weights and solution concentrations of a given polymer and solvent system

2. Photoresist Application

Sample

ResistT

Extra resist at the edges

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3. Soft Bake

Partial evaporation of photo-resist solvents

Improves adhesion Improves uniformity Improves etch resistance Improves linewidth control Optimizes light absorbance

characteristics of photoresist

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4. Alignment and Exposure

Transfers the mask image to the resist-coated wafer

Activates photo-sensitive components of photoresist

Quality measures: linewidth resolution overlay accuracy particles & defects

UV Light Source

Mask

ResistResistResistResist

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Alignment errors (many different types)

Mask aligner equipment Double sided alignment

especially important in micromachines


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5. Develop

Soluble areas of photoresist are dissolved by developer chemical

Visible patterns appear on wafer windows islands

Quality measures: line resolution uniformity particles & defects

to vacuum pump

vacuum chuck

spindle

developerdispenser

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CLEAN ROOM TRAINING 2009-2010. Semester II

6. Hard Bake

Evaporate remaining photoresist

Improve adhesion

Higher temperature than soft bake

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7. Development Inspection

Optical or SEM metrology Quality issues:

particles defects critical dimensions linewidth resolution overlay accuracy

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8. Plasma Etch-Or Add Layer

Selective removal of upper layer of wafer through windows in photoresist: subtractive

Two basic methods: wet acid etch dry plasma etch

Quality measures: defects and particles step height selectivity critical dimensions

Adding materials (additive) Two main techniques:

Sputtering evaporation

PlasmaPlasma

CF4CF4

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9. Photoresist Removal (strip)

No need for photoresist following etch process

Two common methods: wet acid strip dry plasma strip

Followed by wet clean to remove remaining resist and strip byproducts

O2O2

PlasmaPlasma

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10. 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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Summary


Resists

Part ii


Part ii

Miniaturization


Part i

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silicon substrate

oxide

photoresist

Positive Lithography

Island

Window

Areas exposed to light become photosoluble.





photoresist

silicon substrate

oxide

Ultraviolet Light

Positive Resist Tone

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Photoresists

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Photoresist profiles Overcut (LIFT-OFF) Vertical Undercut Dose : High

Developer: Low

Dose : Medium

Developer: Moderate

Dose : Low

Developer: Dominant

Photoresist profiles

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Optical Resolution

PhotosensitivityRefractive Index

Mechanical/Chemical ViscosityAdhesionEtch resistanceThermal stability

Process related Cleanliness (particle count)

Metal ContentShelf lifeToxicityStability to process variations

Photoresist Material Parameters (requirements)

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Positive tone photoresist: DQN

Resin (N) / sensitizer(DQ)

N: phenolic Novolak resin: low molecular weight polymer. Forms the resists films properties. It dissolves in presence of water.

DQ (Photoactive siazoquinone ester) Photosensitive, insoluble in aqueous solution. Prevents the resin to be dissolved

Upon exposure to light, the dizaoquinones photochemically decompose

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Example: AZ 1500 Photoresists

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Positive tone photoresist: PMMA

PMMA: poly(methylmethacrylate)

Chain scission under DUV exposition

Also suitable for electron-beam lithography

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Example: nano-PMMA

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Negative tone photoresist

Resin: Cyclic Synthetic rubber (non radiaton sensitive, strongly soluble in the solvent)

PAC is a bis-arylazide. Upon exposure, it dissociates into nitrene and N2. The nitrene reacts with the rubber molecules so that a cross linking between resin molecules occurs, becoming unsoluble.

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Example: AZ-N4035

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O

CH2

CH

CH2O

Epoxy based negative photoresistNegative photoresists becomeinsoluble in developing solutionswhen exposed to optical radiation

SU-8 is a commercial name for a fixed formulation. Any variation of thisformulation becomes a very similar resist, but as it is not exactly SU-8, thevariations are called epoxy based resists.

C CH3H3C

CH2

C CH3H3C

CH2

C CH3H3C

CH2

C CH3H3C

O O O

OOOO

CH2 CH2 CH2 CH2

CH2CH2

O

CH2 CH2

CH CH CH CH

CHCH CH CH

CH2 CH2 CH2 CH2

CH2CH2CH2CH2

O O O O

O O O O

• On exposure the PAG generates a strong acid

• Protons attack oxygen on some epoxides

• Crosslinking occurs during PEB resulting in an insoluble very dense polymer network

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Example: SU-8

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Concepts and principles of optical Xip lithography - · PDF fileConcepts and principles of optical lithography Francesc Pérez-Murano Institut de Microelectrònica de Barcelona (CNM-IMB,