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Lecture 4: Lithography 2

Prasanna S. GandhiAssistant Professor,Department of Mechanical Engineering,Indian Institute of Technology, Bombay,

MEMS: Fabrication

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Recap: Last Class

LithographyOptical lithography

Contact printing Proximity printingProjection printing

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Today’s Class

E-beam lithographyX-ray lithographyIon beam lithographyOxidationSilicon wafer preparation processClean room fundamentals

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E-Beam Lithography

Features are written by scanning 10-50keV electron beamNo necessity of maskCan be used for preparation of maskVery fine size (sub-micron or <1 micron ~ 20nm) features can be produced easily no diffraction limit: limitation due to electron scatterNot suitable for higher length featuresDeveloped in 1960s: SEM technology

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E-Beam Lithography

Mask making for optical lithographyDirect writing of ICsOpto-electronic devices, Quantum structures, Research applications:

Enhancement of contactCNT probe growth using Ebeam

Applications

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System DescriptionAn electron gun or electron source that supplies the electrons; An electron column that 'shapes' and focuses the electron beam; A mechanical stage that positions the wafer under the electron beam; A wafer handling system that automatically feeds wafers to the system and unloads them after processing; and A computer system that controls the equipment.

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Electron Gun

http://www.elettra.trieste.it/experiments/beamlines/lilit/htdocs/people/luca/tesihtml/node41.html

Cathode: Thermionicemmitter: tungstonhairpin, LaB6 OR field emmiters:

sintered material or crystal

Schottky emmitters

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Electronic GunM/c Electron Source

© FEI Beam Technology 2004

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E-Beam LithographyElectron Sources

100 hrs1000 hrs>1 year>1 yearTypical ServiceLife (hrs)

<1<14 - 6 <1Short-Term Beam Current Stability (%RMS)

1061071095 x 108Brightness (A/cm2SR

1.01.00.2 - 0.30.3 - 1.0Energy Spread (eV)

>104104315Source Size (nm)

TUNGSTENLaB6COLD FIELD

SCHOTTKY

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E-Beam Lithography

Schottky emmittersFor SEM of special resolution

Optics*

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E-Beam Lithography

Scanning *Raster scanVector scanOnce i is set, exposure is controlled by varying speed v and scan spacing s

Stepping:F = 0.25 to 6mmStage movement for scanning the next field

JEOL EBL , Raith, machineVariable beam shape m/c available

M/c: scanning

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E-Beam Lithography

Both positive and negative PRsExposure dose charge/cm2

Parameter γ: slope of thickness vsexposure curve * Resolution depends on electron scatter, better for smaller thicknessPMMA + (γ=2), COP - (Mead Tech) (γ=0.8)

E-beam resists

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Table: Negative and Positive ResisitsLithography Name Type Sensitivity γOptical Kodak 747 Negative 9 mJ/cm2 1.9

AZ-1350J Positive 90 mJ/cm2 1 .4PR102 Positive 140 mJ/cm2 1.9

e-beam COP Negative 0.3 µC/cm2 0.45GeSe Negative 80 µC/cm2 3.5PBS Positive 1 µC/cm2 0.35PMMA Positive 50 µC/cm2 1.0

X-ray COP Negative 175 mJ/cm2 0.45DCOPA Negative 10 mJ/cm2 0.65PBS Positive 95 mJ/cm2 0.5PMMA Positive 1000 mJ/cm2 1.0

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E-Beam Lithography

Line doseFor small scale fine featuresSpacing 100 ALow energy dose ~ 1.5nC/cm2

Area dose For bigger featuresSpacing 100 AHigh energy dose ~ 250 µC/cm2

Dose for PMMA

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SEM Images

Circular Gratings Rose

MEMS Device Radial Dots

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SEM Images

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Concept of Gray Exposure

Structure with varying dose More intensity/dose in areas requiring anchorsLess in areas requiring release

Structures that can be formedFilters, microchannels, polymer accelerometers, mechanisms

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Mechanical logic gate formed by Gray Exposure

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X-ray Lithography

High aspect ratio structuresOptical materials opaque to small wavelengths but transparent to x-raysAll electron resists are also x ray resist, because photoelectrons produced during x-ray absorptionPMMA resist is usually usedX-ray masks different from cr optical masks: e.g. Gold with thickness 0.7µm, 0.5µm, 0.2µm for l 4.4A (Pd), 8.3A (Al), 13.3A (Cu). Metal is thicker than crMask substrates?? Polyamide, SiC, Si3N4, Al2O3

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Ion-beam Lithography

Better than electron beam in terms of resolution low scatter of ionsResists PMMAPerceived as a ‘next generation’ lithography process

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Oxidation

Oxidation of Si*: keep in air at high temp (1000-1200oC)Well understood and controlled processDry and wet oxidation

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Oxidation

ParametersTemperatureEnvironmentTime

Oxide uses from MEMS perspectiveSacrificial layer Important patterning material

Problems: thermal stresses

Bdt

dTAT

BAtBATT

oxox

oxox

=+

+=+

2

Constants,)(2 τ

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Oxidation

ParametersTemperatureEnvironmentTime

At smaller thicknessAt higher thicknessInitial time τ corresponds to initial oxide thickness

2 ( ), Constants

(2 )

ox ox

oxox

T AT B tA B

dTT A Bdt

τ+ = +

+ =

)(/ τ+= tABTox

)( τ+= tBTox

0

10

20

2

2

DN

Nk

Ddds

+

=τ

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Oxidation

Parameters

0

10

20

2

2

DN

Nk

Ddds

+

=τ

016 2 0

016

initial oxide layer (200A in dry oxdn, 0 for wet)

Diffusivity of oxide in Si e.g. D 4 4 10 / at 900Surface reaction rate constant Conc. of oxygen molecules in carrier gas

5.2X10

s

d

D . X cm s CkN

m

−

=

= ===

= 3 02

122 3

2 2

/ in dry O at 1000 and 1 atm No of oxidizing species in the oxide

2.2X10 SiO / in dry O

olecules cmN

molecules cm

=

=

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Oxidation

Parameters

Knowing thickness by observing the color (rough estimate)

bTaB

bTaAB

+′=

+′=

)ln(

log

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Ruska 1987, Madou 1997, and van Zant 1997.

Table Color of silicon dioxide layers of selected thicknessSiO2LayerThicknes, 0.275 0.310µm 0.050 0.075 0.465 0.493 0.50 0.375 0.390

Color Tan Brown Red- Blue Green to Green- YellowViolet yellow- yellow

green

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Conclusions

E-beam lithography: high precision applications, mask preparationX-Ray lithography: expensive and hazardous useful for high aspect ratioIonbeam lithography: Better resolution than e-beam possibleOxidation

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Next class

Si wafer preparationClean room fundamentalsChemical etching processAnisotropic Etching

The following class: Plasma processes