module 1c
DESCRIPTION
micro fabrication techniqueTRANSCRIPT
MME 693Materials Science Technologies for
Applications in Life Sciences
Microfabrication Techniques
Instructor: Vivek VermaMME 693: Materials Science Technologies for Applications in Life Sciences
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Wet-Bulk Surface Micromachining• Features are sculpted in bulk• Wet-bulk machining can be used for
– Cleaning– Shaping three dimensional structures– Removing surface damage– Polishing
• Silicon wafers– Aspect ratio
• Microelectronics 1:2• MEMS 1:400
– 4 inch wafer: 525 μm– 6 inch wafer: 650 μm
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Wet-Bulk Surface Micromachining• Miller indices• Planes are described with parenthesis
– (100), (110), (111), (120)
Saliterman Fundamentals of BioMEMS and Medical Microdevices
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Wet-Bulk Surface Micromachining• Miller indices• Planes are described with parenthesis
– (100), (110), (111), (120)
Saliterman Fundamentals of BioMEMS and Medical Microdevices
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Wet-Bulk Surface Micromachining• Miller indices• Planes are described with paranthesis
– (100), (110), (111), (120)– Set of equivalent directions is described with braces{110}
• Particular direction is described with square bracket– [100] normal to plane (100)– Set of equivalent directions are designated withangle brackets <100>
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Wet-Bulk Surface Micromachining• Various planes in a {100}-orientation wafer
Saliterman Fundamentals of BioMEMS and Medical Microdevices
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Isotropic and Anisotropic Etching• Isotropic etchants are applied to all crystallographic
directions at the same rate–Usually acid etchants– Lead to rounded features
Saliterman Fundamentals of BioMEMS and Medical Microdevices
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Isotropic and Anisotropic Etching
Saliterman Fundamentals of BioMEMS and Medical Microdevices
• Anisotropic etching rates depend on exposed crystal orientation
– Specific orientations get etched much faster– Alkaline materials are used at anisotropic etchants
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Isotropic and Anisotropic Etching
Saliterman Fundamentals of BioMEMS and Medical Microdevices
• Isotropic agents are diffusion limited• Anisotropic agents are rate limited• Isotropic and anisotropic agents involve in oxidation of
silicon followed by dissolution of the hydrated silicate
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Isotropic and Anisotropic Etching• Isotropic etching
Si + HNO3 + 6HF → H2SiF6 + HNO2 + H2O + H2• Isotropic etching is used for
– Removal of work-damaged surfaces– Rounding sharp corners to avoid stress
concentration– Removing roughness
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Isotropic and Anisotropic Etching• Anisotropic etching takes place in hydroxide groups:
• Anisotropic etching results in geometric shapesbounded by the slowest etching plane
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Isotropic and Anisotropic Etching
Saliterman Fundamentals of BioMEMS and Medical Microdevices
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Isotropic and Anisotropic Etching
Saliterman Fundamentals of BioMEMS and Medical Microdevices
• [100]-orientation silicon wafer has inward sloping walls of 54.74°
• [110]-orientation hasfastest etch rate
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Selection of Silicon Wafer Orientation
Saliterman Fundamentals of BioMEMS and Medical Microdevices
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Isotropic and Anisotropic Etching
• Parameters include–Undercutting (bias)– Tolerance– Etch rate– Anisotropy– Selectivity–Over etch– Feature size control– Loading effects
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3D Structures with Sacrificial Layers• Micromachining is used
• Sacrificial layer is used that can be etched away to leave undercut features
– Cantilever parts– Free moving masses– Bridges– Diaphragms
• In 3D surface micromachining, features are built up layer by layer
– Dry etching defines features in x-y plane– Wet etching releases them from the plane by undercutting
• Shapes are restricted by crystallography of the substrate• Example of micromachining
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3D Structures with Sacrificial Layers
http://www.swri.edu/3pubs/ttoday/winter04/images/page9.jpg
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3D Structures with Sacrificial Layers
http://www.sfu.ca/immr/gallery/crm52-01/hinge_3g_2.jpg
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LIGA
• LIGA (German)– Lithographie,
Galvanoformung,Abformung
– Lithography electroplatingmolding
– High aspect ratio process
• http://en.wikipedia.org/wiki/LIGA
• High energy synchrotronX-Ray or UV to pattern
Saliterman Fundamentals of BioMEMS and Medical Microdevices
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LIGA
Saliterman Fundamentals of BioMEMS and Medical Microdevices
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LIGA
Saliterman Fundamentals of BioMEMS and Medical Microdevices
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LIGA
Saliterman Fundamentals of BioMEMS and Medical Microdevices
MME 693: Materials Science Technologies for Applications in Life Sciences 23
LIGA
Saliterman Fundamentals of BioMEMS and Medical Microdevices
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LIGA
Saliterman Fundamentals of BioMEMS and Medical Microdevices
Deep Reactive Ion Etching (DRIE)
• Used for building high aspect ratio micro-machined parts
• 20:1 aspect ratio is nicely achieved• Near vertical walls• Inductively coupled plasma• Bosch process
– Alternating etching and passivation process– Allows deeply etched trenches
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Deep Reactive Ion Etching (DRIE)
Saliterman Fundamentals of BioMEMS and Medical Microdevices
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Deep Reactive Ion Etching (DRIE)
Saliterman Fundamentals of BioMEMS and Medical Microdevices
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Deep Reactive Ion Etching (DRIE)
• SF6 plasma is used for etching
• C4F8 is used for passivation
• DRIE can be used
– In silicon for microfluidic devices
– Producing nanocapillaries
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HEXIL Process
Saliterman Fundamentals of BioMEMS and Medical Microdevices
Silicon walls are wet etched to create smooth surface
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HEXIL Process
Saliterman Fundamentals of BioMEMS and Medical Microdevices
Phosphosilicate glass (PSG) is applied as sacrificial layer
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HEXIL Process
Saliterman Fundamentals of BioMEMS and Medical Microdevices
• Polysilicon is deposited using CVD– Annealing and polishing
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HEXIL Process
Saliterman Fundamentals of BioMEMS and Medical Microdevices
• Another structural (polysilicon) layer is patterned and deposited such that it physically connects with first layer
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HEXIL Process
Saliterman Fundamentals of BioMEMS and Medical Microdevices
• Sacrificial layer is removed using hydrofluoric acid