MEMS: Fabrication

Lecture 4: Lithography 2

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

Recap: Last Class

LithographyOptical lithography

Contact printing Proximity printingProjection printing

Today’s Class

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

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

E-Beam LithographyApplications

Mask making for optical lithographyDirect writing of ICsResearch applications:

Nano-manipulator / probe growth using CNTFine structures

E-Beam Lithography

Cathode: Thermionicemmitter: tungstonhairpin, LaB6 OR field emmiters: sintered material or crystalSchottky emmitters

M/c Electron Source

© FEI Beam Technology 2004

E-Beam LithographyElectron Sources

SCHOTTKY COLD FIELD

LaB6 TUNGSTEN

Source Size (nm)

15 3 104 >104

Energy Spread (eV)

0.3 - 1.0 0.2 - 0.3 1.0 1.0

Brightness (A/cm2SR

5 x 108 109 107 106

Short-Term Beam Current Stability (%RMS)

E-Beam LithographyOptics*

Schottky emmittersFor SEM of special resolution

E-Beam LithographyM/c: scanning

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 machineVariable beam shape m/c available

E-Beam LithographyE-beam resists

Both positive and negativeExposure 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 LithographyDose for PMMA

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

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

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

Ion-beam Lithography

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

Oxidation

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

Oxidation

ParametersTemperatureEnvironmentTime

Oxide uses from MEMS perspectiveSacrificial layer Important patterning material

Problems: thermal stresses

Bdt

dTAT

BAtBATT

oxox

oxox

=+

+=+

2

Constants,)(2 τ

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

Next class

Si wafer preparationClean room fundamentalsChemical etching processAnisotropic Etching

The following class: Plasma processes

Lecture 4: Lithography 2