MEMS Technology Visvesvarya Technological University, Belgaum A Seminar Report On MEMS TECHNOLOGY Submitted in fulfillment for the award of Bachelor of Engineering In Electronics and Communication Engineering Madhura S M (1BM07EC054) Under the guidance of Mr Dinesh Lecturer, Dept. of E&C, BMSCE -1 -
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MEMS Technology
Visvesvarya Technological University, Belgaum
A Seminar Report On
MEMS TECHNOLOGY
Submitted in fulfillment for the award of
Bachelor of Engineering In
Electronics and Communication Engineering
Madhura S M (1BM07EC054)
Under the guidance ofMr Dinesh
Lecturer, Dept. of E&C,BMSCE
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MEMS Technology
CERTIFICATE
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
B.M.S COLLEGE OF ENGINEERING
BANGALORE – 560019
This is to certify that the seminar entitled MEMS Technology has been carried out by Madhura S M bearing USN 1BM07EC054 submitted in the fulfillment for the award of Bachelor of Engineering degree prescribed by the Visvesvaraya Technological University, Belgaum during academic year 2011 .
Seminar Guide Signature Signature of HOD
DATE:
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Contents
Sl No Section/Topic Page No
1. Introduction/overview 4
2. MEMS Description 7
3. MEMS Design Process 8
4. MEMS Fabrication Technologies 17
5. Key applications 19
6. Advantages & comparisons 20
7. Current Challenges 21
8. Future Developments 22
9. Conclusion 23
10. Refrences 24
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ABSTRACT
The technology, Micro-Electro-Mechanical-Systems (MEMS), emerged in the
late1980s which enables us to fabricate mechanical parts on the order of microns.
Micromachining technology is suitable for developing new transducers or improving
existing transducer designs. Due to the dramatic reduction in size, micro transducers
can outperform traditional ones by orders of magnitude. Furthermore, MEMS is a
fundamental technology which has the potential to influence advancements in many
fields. In the automobile, electronics, bio-medical and television industries, MEMS
products have already made appreciable impacts.
SECTION 1 INTRODUCTION
Microelectromechanical systems (MEMS) are small integrated devices
or systems that combine electrical and mechanical components. They range in size
from the sub micrometer level to the millimeter level and there can be any number,
from a few to millions, in a particular system. MEMS extend the fabrication
techniques developed for the integrated circuit industry to add mechanical elements
such as beams, gears, diaphragms, and springs to devices.
Examples of MEMS device applications include inkjet-printer cartridges,
microtransmissions, micromirrors, micro actuator (Mechanisms for activating process
control equipment by use of pneumatic, hydraulic, or electronic signals) optical
scanners, fluid pumps, transducer, pressure and flow sensors. New applications are
emerging as the existing technology is applied to the miniaturization and integration
of conventional devices.
These systems can sense, control, and activate mechanical processes on the micro
scale, and function individually or in arrays to generate effects on the macro scale.
The micro fabrication technology enables fabrication of large arrays of devices, which
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MEMS Technology
individually perform simple tasks, but in combination can accomplish complicated
functions.
SECTION 1.1 WHAT IS MEMS TECHNOLOGY?
Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical
elements, sensors, actuators, and electronics on a common silicon substrate through
microfabrication technology. While the electronics are fabricated using integrated
circuit (IC) process sequences, the micromechanical components are fabricated using
compatible "micromachining" processes that selectively etch away parts of the silicon
wafer or add new structural layers to form the mechanical and electromechanical
devices.
Microelectronic integrated circuits can be thought of as the "brains" of a system and
MEMS augments this decision-making capability with "eyes" and "arms", to allow
microsystems to sense and control the environment. Sensors gather information from
the environment through measuring mechanical, thermal, biological, chemical,
optical, and magnetic phenomena. The electronics then process the information
derived from the sensors and through some decision making capability direct the
actuators to respond by moving, positioning, regulating, pumping, and filtering,
thereby controlling the environment for some desired outcome or purpose. Because
MEMS devices are manufactured using batch fabrication techniques similar to those
used for integrated circuits, unprecedented levels of functionality, reliability, and
sophistication can be placed on a small silicon chip at a relatively low cost.
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SECTION 1.2 WHAT ARE MEMS / MICROSYSTEMS?
As the smallest commercially produced "machines", MEMS devices are
similar to traditional sensors and actuators although much, much smaller. E.g.
Complete systems are typically a few millimeters across, with individual features
devices of the order of 1-100 micrometers across.
MEMS devices are manufactured either using processes based on Integrated Circuit
fabrication techniques and materials, or using new emerging fabrication technologies
such as micro injection molding. These former processes involve building the device
up layer by layer, involving several material depositions and etch steps. A typical
MEMS fabrication technology may have a 5 step process. Due to the limitations of
this "traditional IC" manufacturing process MEMS devices are substantially planar,
having very low aspect ratios (typically 5 -10 micro meters thick). It is important to
note that there are several evolving fabrication techniques that allow higher aspect
ratios such as deep x-ray lithography, electrodeposition, and micro injection molding.
MEMS devices are typically fabricated onto a substrate (chip) that may also contain
the electronics required to interact with the MEMS device. Due to the small size and
mass of the devices, MEMS components can be actuated electrostatically
(piezoelectric and bimetallic effects can also be used). The position of MEMS
components can also be sensed capacitively. Hence the MEMS electronics include
electrostatic drive power supplies, capacitance charge comparators, and signal
conditioning circuitry. Connection with the macroscopic world is via wire bonding
and encapsulation into familiar BGA, MCM, surface mount, or leaded IC packages.
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A common MEMS actuator is the "linear comb drive" (shown above) which consists
of rows of interlocking teeth; half of the teeth are attached to a fixed "beam", the other
half attach to a movable beam assembly. Both assemblies are electrically insulated.
By applying the same polarity voltage to both parts the resultant electrostatic force
repels the movable beam away from the fixed. Conversely, by applying opposite
polarity the parts are attracted. In this manner the comb drive can be moved "in" or
"out" and either DC or AC voltages can be applied. The small size of the parts (low
inertial mass) indicates that the drive has a very fast response time compared to its
macroscopic counterpart. The magnitude of electrostatic force is multiplied by the
voltage or more commonly the surface area and number of teeth. Commercial comb
drives have several thousand teeth, each tooth approximately 10 micro meters long.
Drive voltages are CMOS levels.
The linear push / pull motion of a comb drive can be converted into
rotational motion by coupling the drive to push rod and pinion on a wheel. In this
manner the comb drive can rotate the wheel in the same way a steam engine
functions!
SECTION 2 MEMS DESCRIPTION
MEMS technology can be implemented using a number of different
materials and manufacturing techniques; the choice of which will depend on the
device being created and the market sector in which it has to operate.
SILICON
The economies of scale, ready availability of cheap high-quality materials and ability
to incorporate electronic functionality make silicon attractive for a wide variety of
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MEMS applications. Silicon also has significant advantages engendered through its
material properties. In single crystal form, silicon is an almost perfect Hookean
material, meaning that when it is flexed there is virtually no hysteresis and hence
almost no energy dissipation. The basic techniques for producing all silicon based
MEMS devices are deposition of material layers, patterning of these layers by
photolithography and then etching to produce the required shapes.
POLYMERS
Even though the electronics industry provides an economy of scale for the silicon
industry, crystalline silicon is still a complex and relatively expensive material to
produce. Polymers on the other hand can be produced in huge volumes, with a great
variety of material characteristics. MEMS devices can be made from polymers by
processes such as injection moulding, embossing or stereolithography.
METALS
Metals can also be used to create MEMS elements. While metals do not have some of
the advantages displayed by silicon in terms of mechanical properties, when used
within their limitations, metals can exhibit very high degrees of reliability. Metals can
be deposited by electroplating, evaporation, and sputtering processes.
SECTION 3 MEMS DESIGN PROCESS
There are three basic building blocks in MEMS technology - Deposition Process-the
ability to deposit thin films of material on a substrate, Lithography-to apply a
patterned mask on top of the films by photolithograpic imaging, and Etching-to etch
the films selectively to the mask. A MEMS process is usually a structured sequence of
these operations to form actual devices.
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SECTION 3.1 DEPOSITION PROCESSES
One of the basic building blocks in MEMS processing is the ability to deposit thin
films of material. MEMS deposition technology can be classified in two groups:
1. Depositions that happen because of a chemical reaction:
o Chemical Vapor Deposition (CVD)
o Electrodeposition
o Epitaxy
o Thermal oxidation
2. Depositions that happen because of a physical reaction:
o Physical Vapor Deposition (PVD)
o Casting
CHEMICAL VAPOR DEPOSITION (CVD)
In this process, the substrate is placed inside a reactor to which a number of gases are supplied. The fundamental principle of the process is that a chemical reaction takes place between the source gases. The product of that reaction is a solid material with condenses on all surfaces inside the reactor. The two most important CVD technologies in MEMS are the Low Pressure CVD (LPCVD) and Plasma Enhanced CVD (PECVD). The LPCVD process produces layers with excellent uniformity of thickness and material characteristics. The main problems with the process are the
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high deposition temperature (higher than 600°C) and the relatively slow deposition rate. The PECVD process can operate at lower temperatures (down to 300° C) thanks to the extra energy supplied to the gas molecules by the plasma in the reactor. However, the quality of the films tends to be inferior to processes running at higher temperatures.
Figure 1: Typical hot-wall LPCVD reactor.
ELECTRODEPOSITION
This process is also known as "electroplating" and is typically restricted
to electrically conductive materials. There are basically two technologies for plating:
Electroplating and Electro-less plating. In the electroplating process the substrate is
placed in a liquid solution(electrolyte). When an electrical potential is applied
between a conducting area on the substrate and a counter electrode (usually platinum)
in the liquid, a chemical redox process takes place resulting in the formation of a layer
of material on the substrate and usually some gas generation at the counter electrode.
In the electro-less plating process a more complex chemical solution is
used, in which deposition happens spontaneously on any surface which forms a
sufficiently high electrochemical potential with the solution. This process is desirable
since it does not require any external electrical potential and contact to the substrate
during processing. Unfortunately, it is also more difficult to control with regards to
film thickness and uniformity. A schematic diagram of a typical setup for
electroplating is shown in the figure below.
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EPITAXY
This technology is quite similar to what happens in CVD processes,
however, if the substrate is an ordered semiconductor crystal (i.e. silicon, gallium
arsenide), it is possible with this process to continue building on the substrate with the
same crystallographic orientation with the substrate acting as a seed for the
deposition. If an amorphous/polycrystalline substrate surface is used, the film will
also be amorphous or polycrystalline.
There are several technologies for creating the conditions inside a
reactor needed to support epitaxial growth, of which the most important is Vapor
Phase Epitaxy (VPE). In this process, a number of gases are introduced in an
induction heated reactor where only the substrate is heated. The temperature of the
substrate typically must be at least 50% of the melting point of the material to be
deposited. A schematic diagram of a typical vapor phase epitaxial reactor is shown in