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• What is LIGA?
• The LIGA Process
• Lithography Techniques
• Electroforming
• Molding (Mold Fabrication)
• Analyzing Processing Problems
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
• 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
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 ProcessDevelopment
PMMA Surface Analysis
PMMA patterned usingTitanium Membrane Mask
PMMA patterned usingGraphite Membrane Mask
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
Application of Aligned MoldingLIGA Acceleration Sensor
PMMA structure Ni - structures 120 µm high
Redundant Sensor System
Detail
‘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
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