Surface micromachining
-
Upload
gurunath-apte -
Category
Engineering
-
view
180 -
download
9
Transcript of Surface micromachining
SURFACE MICROMACHINING
NIKITA JAVIA
GURUNATH APTE
What is micromachining?
• Micromachining is used to fabricate three-dimensional
microstructures and it is the foundation of a technology called
Micro-Electro-Mechanical-Systems (MEMS).
• Micromachining is the basic technology for fabrication of micro-
components of size in the range of 1 to 500 micrometers.
• Their need arises from miniaturization of various devices in
science and engineering, calling for ultra-precision manufacturing
and micro-fabrication.
• Bulk micromachining and surface micromachining are two major
categories in this field.
Surface micromachining
• Surface micromachining builds microstructures by deposition and
etching of different structural layers on top of the substrate.
• Generally polysilicon is commonly used as one of the layers and
silicon dioxide is used as a sacrificial layer which is removed or
etched out to create the necessary void in the thickness direction.
• Added layers are generally very thin with their size varying from 2-5
Micro metres.
• The size of the substrates can also be much larger than a silicon wafer,
and surface micromachining is used to produce TFTs on large area
glass substrates for flat panel displays.
Advantages• The main advantage of this machining process is the
possibility of realizing monolithic microsystems in which the
electronic and the mechanical components(functions) are built
in on the same substrate.
• The surface micromachined components are smaller in
thickness and mass.
• The expensive silicon wafers can be replaced by cheaper
substrates, such as glass or plastic.
• It is cost effective.
Disadvantages
• Multiple deposition and etching required to build up
structures.
• Vertical dimensions are limited to the thickness of the
deposited layers leading to compliant suspended structures
with tendency to stick support
• Cleanliness is critical at end of process.
• Sawing, packing and testing is difficult.
APPLICATIONS
• Used in manufacturing of flat panel television screen
• Used in production of thin solar cells
• Used in making bimetal cantilever used for monitoring
mercury vapour, moisture, protein conformational changes in
antigen antibody binding
ExamplesSurface Micromachining can be seen in action in the following
MEMS products:
• Surface Micromachined Accelerometers.
• 3D Flexible Multichannel Neural Probe Array.
• Nanoelectromechanical relays
Fabrication Process• Micromachining starts with a silicon wafer or other substrate
and grows layers on top.
• These layers are selectively etched by photolithography and
either a wet etch involving an acid or a dry etch involving an
ionized gas, or plasma.
• Dry etching can combine chemical etching with physical
etching, or ion bombardment of the material.
• Surface micromachining can involve as many layers as is
needed with a different mask on each layer.
• Surface micromachining uses developed technology which is
very repeatable for volume production.
STUCTURAL LAYER• The layer of thin film material with which the microstructures are made of.
• Has physical and chemical properties that are suitable for the desired
application
• Mechanical properties such as high yield and fracture stresses, minimal
creep and fatigue, and good wear resistance
• Polysilicon is usually used as a structural material.
Disadvantages of poly Si (over single crystal Si)
• Lower yield strength
• Lower piezo resistivity
• Stiction.
Sacrificial Layer
• The layer of material used during the fabrication process to
deposit microstructures.
• These are removed towards the end of the fabrication. So, the
layer has no role in the operation of the device.
• Good mechanical properties so that device does not fail while
fabrication
• Good adhesion
• Low residual stresses
Basic Sacrificial Layer Processing
Example: Scanning Probe
Microscopy
• Problems with existing processes
o Etching of positive pyramids
• Difficult tot control etch stop point
• uniformity difficult to obtain
o Bulk etching
• Long etching time involved to etch through
the wafer
A hybrid method to fabricate SPM probes
THANK YOU