LIGA Process

Prof. Tianhong Cui

ME 8254

Microfabrication Technology

Outline

• What is LIGA?

• The LIGA Process

• Lithography Techniques

• Electroforming

• Molding (Mold Fabrication)

• Analyzing Processing Problems

What is LIGA ?

LI thographie Lithography

G alvanoformung Electroforming

A bformung Moulding

The LIGA Process• Lithography

In general terms lithography is an imagetransfer process using …….

Visible and UV LightElectron BeamIon BeamLaserMachiningX-ray

• For LIGA

The key consideration is high aspect ratiostructures are required

High Aspect RatioPatterning

Height: ~250 µmWidth: ~ 14 µm

Height: ~360 µmWidth: ~ 14 µm

Specialized OpticalLithography Processes

SU-8Resist

X-ray LithographyShadow Printing Using X-rays

• X-ray mask• Resist• Substrate

• Development

• Arbitrary Shape

• Structure Height up to Several Millimeters

• Minimum Feature Sizes in the Order of Micrometers

• Sub-micrometer Topographical Details

• Vertical Sidewall Profile

• Smooth Sidewalls

Key Features of DXRLMicrostructures

Two Beam Transport Lines and Scannersfor Deep X-ray Lithography

Deep X-ray Lithography

Mask Materials:

• Handling ring – Pyrex, glass, metal …

• Carrier – Si(B), SiC, SiN, Si, Be, Ti, C …thickness – from ~2-5 µm up to 200 µm

• Absorber – Au, W(Si,N), Ta(Si,N)thickness – from ~0.5-1 µm up to 50 µm

X-Ray MaskExposure

G

Abso

rbed

Dos

e

Peak Dose

DevelopmentLevel

w

Developed Resist

W

X-rays

PlatingBase

NIST Standard MEMS X-Ray MaskDA=100 mm, Dw=100 mm, T=7 mm, L=3 mm,

patterned area up to DI =80 mm

PatternCarrier

HandlingRing

AbsorberPattern

X-ray Mask Membranes for DXRL

• Silicon Based (Si, SiC, Si3N4)

=> Acceptable X-Ray Transmission, Mechanically Stiff, ReasonableOptical Transparency (SiC, Si3N4), Widely Used Material

but: Thin Membrane of 1-3 Micrometers Thickness, Reduced ThermalConduction Characteristics

• Titanium

=> Acceptable X-Ray Transmission and Stiffness

but: Thin Membrane of 2-3 Micrometers Thickness, Poor ThermalConduction, no Optical Transparency

• Beryllium=> Excellent X-Ray Transmission, Mechanically Stable Substrate,

Good Thermal Conduction for Mask Cooling

but: Potentially Toxic, not Optically Transparent, High Cost

• Diamond=> Reasonable X-Ray Transmission, Mechanically Stable,

Good Thermal Conduction, Optically Transparent

but: Free Standing Membrane in Required Size Range Difficult toFabricate, High Cost

• Rigid Graphite

=> Reasonable X-Ray Transmission, Rigid, Mechanically StableSubstrate, Good Thermal Conduction, Off-the-Shelf

but: Bulk Porosity, Surface Roughness, not Optically Transparent

X-ray Mask Membranes for DXRL

X-ray Mask FabricationIntermediate Mask Technique

X-ray Mask FabricationOptical Lithography

X-ray Mask ProcessDevelopment

Graphite as the MaskMembrane

ElectrodepositedGold Absorber

Exposed Pattern Analysis

Sidewall Roughness

X-ray Mask ProcessDevelopment

PMMA Surface Analysis

PMMA patterned usingTitanium Membrane Mask

PMMA patterned usingGraphite Membrane Mask

X-ray Mask ProcessDevelopment

Materials Analysis

Secondary Effects in DXRL

X-ray Lithography3-Dimensional Patterning

Electroplatingof metal structuresand mold inserts

Replication by molding(hot embossing,

injection molding)

Electroforming and Molding

Optimized Plating ConditionsResults in more UniformDeposition of Structures withDifferent Dimensions.

Electroplating in High AspectRatio Structures - Uniformity

Mold FabricationNickel Electroplating

Application of Aligned MoldingLIGA Acceleration Sensor

PMMA structure Ni - structures 120 µm high

Redundant Sensor System

Detail

Microfluidics• Microfluidics & Process Control

– High-Pressure Micropumps– Micromixers

‘True’ LIGA Drawbacks are

• Complex Process with ‘Too’ Many Process Steps• Need for an X-Ray Synchrotron Source

=> ‘Relatively’ Long Processing Time and High Costs

‘Low-Cost’ LIGA Relies on

• Optical Lithography of Very Thick Negative Resists (SU-8)• Uses only ‘Established’ and ‘Accepted’ Process Equipment• Simple Way to Fabricate Multilevel Microstructures

=> Attractive High Aspect Ratio Microstructures Can be Offeredwith Short Throughput Time and at Low Costs

Optical or Low-Cost HARMSProcess

Low-Cost HARMS Process Scheme“Poor-Man” LIGA

Example ofa MultilevelPositiveStructure

Star PatternedLines in SU-8,

Showing anAspect Ratio of

14.5

CourtesyIBM

http://dmtwww.epfl.ch/ims/micsys/subtopics/projects/su8

Examples of SU-8 Microstructures

Surface Roughness ofPinion TeethInjected in aMIMOTEC Mold asComparedwith a WEDM Mold

2.1 mm HighStructures

Defined in SU-8

3D Springs

• Process Development– SU-8– Electroforming

Mold– Embossing– Sputtering– CMP/Bonding

Fuel Cells– Proton Membranes– Coupled to

Microreactors

Micro-Separator & Concentrator• Adsoprtion/Desorption Separators