Lecture #4
MEMS Fabrication Processes
Dr. Shady Abdelnasser, Ph.D.
Department of Energy and Renewable Energy Engineering
Egyptian Chinese University
Outline
Micro electromechanical Systems (MEMS)
Additive & Subtractive Manufacturing
Manufacturing Steps for Microfabrication
Definitions
Mechatronics is the integration of mechanical
systems with electronic components to create
more functional and efficient products and
processes.
MEMS devices often integrate mechanical and
electrical components on a single chip at
(miniaturized) small-scale systems.
It involves mechanical components like beams,
gears, and sensors, along with electronic
components.
MEMS devices often include (accelerometers,
temperature and pressure sensors), actuators,
gyroscopes for angular velocity detection)
Processes for MEMS
Fabrication
• Additive manufacturing (AM) processes
that build objects by adding material layer by
layer, while subtractive manufacturing (SM)
removes material to create parts.
• AM, is a technology used to manufacture
physical objects by depositing thin layers of
material on top of each other
• SM refers to processes that includes cutting
or etching. It works by subtracting material
from solid stock to make shapes and
components.
MEMS Production
MEMS (microelectromechanical systems) devices are constructed on a microscopic
scale using technologies such as wet and dry etching and thin film deposition onto
Si wafer. Applications such as sensors and optical displays
AM gives less amount of materials waste
Processes for MEMS Fabrication
Surface vs Bulk Micro-Machining
MEMS devices are either built into a
substrate (bulk micromachining) or on a
substrate (surface micromachining)
Bulk micromachining defines structures by
selectively etching inside a substrate.
Surface micromachining builds
microstructures by deposition and etching
structural layers over a substrate. This is
different from Bulk micromachining, in
which a silicon substrate wafer is selectively
etched to produce structures.
Surface Micro-Machining Process
Silicon Wafers
Si Ingots (bars) Unpolished Si
Wafer
• MEMS fabrication processes start with a substrate,
or a wafer.
• Silicon (Si) wafers are used in most cases as
substrates for MEMS devices.
• Wafers are created by cutting the silicon ingot to
Polished Si thin wafers with variable thickness as desired.
Wafer
Wafer Cleaning
The RCA cleaning (Radio Corporation of America) is a
standard set of wafer cleaning steps which needs to be
performed before proceeding with micromachining steps.
It is a cleaning method developed in order to remove both
organic and ionic contaminants from wafers.
RCA cleaning includes the following steps:
❖Removal of the organic contaminants (Organic Cleaning)
❖Removal of thin oxide contaminants (Oxide Stripping)
❖Removal of ionic contamination (Ionic Cleaning)
Wafer Cleaning
Removing Organics
(H2SO4 + H2O2)
(H2O +H2O2 +NH4OH)
The wafers are prepared by
soaking them in DI water, Removing Oxides
distilled water, then (HF+H2O2)
performing the RCA clean
as follows:
Removing ions
(HCl+H2O2+H2O)
Clean wafer and ready for
the fabrication process
Oxidation of Silicon
Deposit sacrificial layer to protect silicon substrate
Oxidation is one of the basic steps in the silicon processing and IC
microelectronics. This layer helps to protect the area of the substrate from the
chemicals used later for machining
Oxygen from different sources (dry or wet) reach the surface of the substrate and
react on it to form SiO2 layer.
The Si substrate surface is covered by
a layer made of silicon dioxide (SiO2)
Oxidation of Silicon
❖ Silicon oxidizes at room temperature in a
normal atmosphere. However, this
oxidation is only a few atoms thick with
very low quality.
❖ Oxidation of silicon occurs at high
temperature, usually above 800°C.
❖ Also, There are two types of oxidation, wet
and dry.
Oxidation of Silicon
In the first reaction a dry process is utilized
involving oxygen gas as the oxygen source and
the second reaction describes a wet process
which uses steam
Dry Oxidation Wet Oxidation
𝑆𝑖 + 𝑂2 → 𝑆𝑖𝑂2 𝑆𝑖 + 2𝐻2 𝑂 → 𝑆𝑖𝑂2 + 2𝐻2
Thermal Oxidation (Dry oxidation): Slow
diffusion of species leads to slow growth rate
And hence, dense oxide formed (i.e. good
quality,)
A layer of Silicon Dioxide (SiO2) will result
Wet oxidation: Species diffuse much faster. from both thermal and wet oxidation of
Relatively porous oxide formed (lower quality) silicon processes on top of the Si wafer.
Patterning
Design Image
A method of transferring a pattern to a flat substrate
Pattern transfer, Lithography
Lithography is the process of transferring
patterns of geometric shapes in a mask to
a thin layer of radiation-sensitive material
(called resist) covering the surface of the
Si substrate.
This process enable the direct transfer of
the mask pattern onto the Si wafer
After the patterns are defined, an etching
process is employed to selectively remove
the underlying layer.
Pattern transfer,
Lithography
• The photolithographic
system contains:
• Illumination source
• Shutter
• Mask with/without an
optical system
• Photosensitive layer
(photoresist)
Pattern transfer, Photoresist
Photoresist is a photosensitive material
(polymer) that is spun on the wafer surface.
A photoresist is a light-sensitive material used
to transfer micro- and nanoscale patterns to a
substrate in processes, called photolithography
to form a patterned coating on a surface.
Thickness of photoresist layer depends on:
Viscosity
Spinning time
Spinning speed
Pattern transfer, Photoresist
Thin film deposition
Depositing thin films on the surface of the substrate.
• Deposited films could be (Silicon, Copper, Titanium,… etc).
• Two deposition methods are used, Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD)
which we can use to deposit thin films on various substrates.
PVD does not require a chemical reaction to deposit CVD involves reacting gas phase chemicals in a
a thin film layer on a Si wafer. It involves removing chamber to form a thin solid film on a substrate.
atoms, molecules, and ions from a solid source It uses precursor gases as sources that chemically
(target) and condensing them onto the substrate. react on or close to the surface of the substrate to
PVD involves bombarding a metal target in a form a solid film.
chamber and re-depositing atoms as a thin film on a
substrate.
Thin film deposition
The key difference between PVD and CVD is that the
coating material in PVD is in solid form whereas in
CVD it is in gaseous form.
Etching
Patterned wafers usually require etching.
It is the process where unwanted areas of the film are removed. Removing the underlying sacrificial
layer without damaging the structural element.
Etching can be classified as wet etching (chemical) or dry etching (plasma etching).
Wet etching involves using liquid chemicals or etchants to remove material (Acids, bases, and other
solvents). Dry etching involves removing material using gases or plasma in a vacuum chamber
Wet (chemical) etching etches are generally more selective than plasma etches
Chemical process leads to isotropic etch whereas, plasma leads to anisotropic etch
Isotropic etching involves removing
material in all directions at an equal
Etching Mechanism rate, creating a rounded profile.
Meanwhile, anisotropic etching
removes material along the vertical
direction, producing sharp corners
and edges
Steps for Surface Micro Machining
MEMS-based cantilever
Fabrication Process, Micromachining
Wafer level process Pattern transfer
Wafer Cleaning Photo-lithogrpahy
Oxidation Masking
Film deposition Photoresist
Etching
Solution of Bonus Problem
Faraday’s law governs the induced force (or a motion) in the wire under the influence of
a magnetic field
Scaling in Electromagnetic Forces
Thus, electromagnetic force
scales as l4.
This indicates, that 10 times
reduction in the plate sizes
(l) means a 10,000 times
decrease in the induced
electromagnetic force
Scaling: A10 times
reduction in size (l) cause
•Electromagnetic force:
10,000 times reduction
•Electrostatic force: only 100
times reduction .
