module 1c

MME 693Materials Science Technologies for

Applications in Life Sciences

Microfabrication Techniques

Instructor: Vivek VermaMME 693: Materials Science Technologies for Applications in Life Sciences

MME 693: Materials Science Technologies for Applications in Life Sciences 2

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


Wet-Bulk Surface Micromachining• Miller indices• Planes are described with parenthesis

– (100), (110), (111), (120)

Saliterman Fundamentals of BioMEMS and Medical Microdevices


Wet-Bulk Surface Micromachining• Miller indices• Planes are described with parenthesis

– (100), (110), (111), (120)



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>


Wet-Bulk Surface Micromachining• Various planes in a {100}-orientation wafer



Isotropic and Anisotropic Etching• Isotropic etchants are applied to all crystallographic

directions at the same rate–Usually acid etchants– Lead to rounded features



Isotropic and Anisotropic Etching


• Anisotropic etching rates depend on exposed crystal orientation

– Specific orientations get etched much faster– Alkaline materials are used at anisotropic etchants




• 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


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


Isotropic and Anisotropic Etching• Anisotropic etching takes place in hydroxide groups:

• Anisotropic etching results in geometric shapesbounded by the slowest etching plane

• [100]-orientation silicon wafer has inward sloping walls of 54.74°

• [110]-orientation hasfastest etch rate


Selection of Silicon Wafer Orientation




• Parameters include–Undercutting (bias)– Tolerance– Etch rate– Anisotropy– Selectivity–Over etch– Feature size control– Loading effects


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


3D Structures with Sacrificial Layers

http://www.swri.edu/3pubs/ttoday/winter04/images/page9.jpg


3D Structures with Sacrificial Layers

http://www.sfu.ca/immr/gallery/crm52-01/hinge_3g_2.jpg


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



LIGA


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




• SF6 plasma is used for etching

• C4F8 is used for passivation

• DRIE can be used

– In silicon for microfluidic devices

– Producing nanocapillaries


HEXIL Process


Silicon walls are wet etched to create smooth surface


HEXIL Process


Phosphosilicate glass (PSG) is applied as sacrificial layer


HEXIL Process


• Polysilicon is deposited using CVD– Annealing and polishing


HEXIL Process


• Another structural (polysilicon) layer is patterned and deposited such that it physically connects with first layer


HEXIL Process


• Sacrificial layer is removed using hydrofluoric acid

module 1c

Documents

materials science technologies

life sciencesmme

anisotropic agents

anisotropic etching

anisotropic etchantsmme

faster alkaline materials

bulk wetbulk machining

medical microdevicesmme