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Micro-Electro-Mechanical Systems (MEMS)

Written By: 

Preeti Jain

Micro-Electro-Mechanical Systems or MEMS Technology is a precision device technology that integrates mechanical elements, sensors, actuators, and electronics on a common silicon substrate through micro fabrication technology.

 

            Micro               :           Small size, microfabricated structures
            Electro             :           Electrical Signal/ Control
            Mechanical      :           Mechanical functionality
            Systems           :           Structures, Devices, Systems
 
MEMS is also referred to as MST (Microsystems Technology in Europe) and MM (Micromachines in Japan). MEMS with optics is called MOEMS- Micro-Opto-Electro-Mechanical-Systems).
A Diagram Illustrating Micro Electro Mechanical System, Opto Electro System, and Micro Opto Electro Mechanical System
Fig. 1: A Diagram Illustrating Micro Electro Mechanical System, Opto Electro System, and Micro Opto Electro Mechanical System
 
ICs can be thought of as the "brains" of a system and MEMS augments it with the “Senses” and “Limbs”.  While the electronics are fabricated using integrated circuit (IC) batch processing techniques, MEMS uses compatible "micromachining" processes, in addition to IC fabrication processes, that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices. These systems can sense, control and actuate on microscale, and function individually or in arrays to generate effects on macroscale.
 
In MEMS devices, a three-dimensional spatial structure is formed on the substrate and mechanical blocks are formed within that structure. A system is then created by fabricating electrical circuits that drive those mechanical blocks on the same substrate. This technology is targeted at devices that range in size from mm down to microns, and involve precision mechanical components that can be manufactured using semiconductor manufacturing technologies. These devices can replace bulky sensors and actuators with micron-scale equivalents. This reduces cost, bulk, weight and power consumption while increasing performance, production volume and functionality.
 
In the most general form, MEMS consist of mechanical microstructures, microsensors, microactuators and microelectronics, all integrated onto the same silicon chip. This is shown schematically in the figure.
A Figure Illustrating Elements of MEMS
Fig. 2: A Figure Illustrating Elements of MEMS
 
Microsensors detect changes in the system’s environment by measuring mechanical, thermal, magnetic, chemical or electromagnetic information or phenomena. Microelectronics processes this information and signals the microactuators to react and create some form of changes to the environment.
An example of a MEMS device is an accelerometer used for car airbags. In addition to micromachined components, these packages typically include signal conditioning circuit, self-testing and calibration, with all the required I/O ports and terminals.

 

WHY MEMS
1.      MEMS allow miniaturization of existing devices
2.      MEMS offer solutions which cannot be attained by macro-machined products, e.g., capacitive pressure sensor capable of sensing pressure of the order of 1 mTorr is not possible with macromachined capacitive diaphragm.
3.      Interdisciplinary nature of MEMS technology and its micromachining techniques, as well as its diversity of applications has resulted in an unprecedented range of devices and synergies across previously unrelated fields (for example biology- microelectronics, optics-microelectronics).
4.      MEMS allows the complex electromechanical systems to be manufactured using batch fabrication techniques, decreasing the cost and increasing the reliability.
5.      It allows integrated systems, viz., sensors, actuators, circuits, etc. in a single package and offers advantages of reliability, performance, cost, ease of use, etc.  
 
DIFFERENCES: ICs Vs MEMS
Though most Si-based MEMS and ICs are fabricated using same microfabrication processes, the two are different on many aspects, some of which are listed in the following table:
S. No
MEMS
ICs
1
3D complex structures
2D structures
2
Doesn’t have any basic building block
Transistor is basic building block of ICs
3
May have moving parts
No moving parts
4
May have interface with external media.
Totally isolated with media
5
Functions include biological, chemical, optical
Only electrical
6
Packaging is very complex
Packaging techniques are well developed.
 

 

Fabrication Techniques

MEMS-FABRICATION TECHNIQUES

MEMS devices use semiconductor processing technologies to produce 3D mechanical structures.

A Figure Showing 3 most used Fabrication Technologies of MEMS

Fig. 3: A Figure Showing 3 Most Used Fabrication Technologies of MEMS

      The three most used fabrication technologies include Bulk Micro Machining, Surface Micro Machining and LIGA
·         BULK MICROMACHINING
In bulk micromachining, the bulk of the substrate, i.e., single crystal silicon, a very stable mechanical material, is specifically removed to form three-dimensional MEMS devices. A Diagram Showing 3-Dimensional Structure of MEMS used in Bulk Micromaching
Fig. 4: A Digram Showing 3-Dimensional Structure of MEMS used in Bulk Micromaching
 
The bulk micromachining manufacture of micro devices generally uses top-down fabrication techniques of etching deep into prepared silicon wafers to create three-dimensional MEMS components. It is a subtractive process that uses wet anisotropic etching or a dry etching method such as reactive ion etching (RIE), to create large pits, grooves and channels. Materials typically used for wet etching include silicon and quartz, while dry etching is typically used with silicon, metals, plastics and ceramics.
·         Wet Etching
In Wet etching, the material is removed through the immersion of a material (typically a silicon wafer) in a liquid bath of a chemical etchant. These etchants can be isotropic (HNA – mixture of HF, HNO3 and Ch3COOH) or anisotropic (KOH). Anisotropic etchants etches faster in a preferred direction; etching is dependent on the crystal orientation of the substrate.
·         Dry Etching
In dry etching, energetic ions are accelerated towards the material to be etched within a plasma phase supplying the additional energy needed for the reaction. The most common form for MEMS is reactive ion etching (RIE) which utilizes additional energy in the form of radio frequency (RF) power to drive the chemical reaction.
·         Deep Reactive Ion Etching(DRIE)
Deep Reactive Ion Etching (DRIE) is a much higher-aspect-ratio etching method that involves an alternating process of high-density plasma etching (as in RIE) and protective polymer deposition to achieve greater aspect ratios
 
The transduction mechanism widely used in bulk micromachined sensors, e.g., pressure senor is the piezoresistive effect. In piezoresistive materials, the change in the stress causes a strain and a corresponding change in the resistance. Thus, when implanted piezoresistors are formed at the maximum stress points of the diaphragm (in case of pressure sensor), the deflection under the applied pressure causes a change in the resistance.
 
·         SURFACE MICROMACHINING
In surface micromachining, the 3-D structure is built up by the orchestrated addition and removal of a sequence of thin film layers to/from the wafer surface called structural and sacrificial layers, respectively.  Sacrificial layers are deposited and then removed to form the mechanical spaces or gaps between the structural layers.  The process steps for surface micromachined cantilever are shown below:
A Figure Showing Different Process Steps for Surface Micromachined Cantilever

 

Fig. 5: A Figure Showing Different Process Steps for Surface Micromachined Cantilever

      Many of the surface micro-machined sensors use the capacitive transduction method to convert the input mechanical signal to the equivalent electrical signal. In the capacitive transduction method, the sensor can be considered a mechanical capacitor in which one of the plates moves with respect to the applied physical stimulus. This changes the gap between the two electrodes with a corresponding change in the capacitance. This change in capacitance is the electrical equivalent of the input mechanical stimulus.
 
·         LIGA
      LIGA is a German acronym consisting of the letters LI (Roentgen Lithography, meaning X-ray lithography), G (Galvanik, meaning electrodeposition) and A (Abformung, meaning molding of other materials into high aspect ratio structures  ). Accordingly, in this technique thick photoresists are exposed to X-rays to produce molds that are subsequently used to form high-aspect ratio electroplated 3-D structures.  The LIGA process can build microparts that are smaller than conventional machining processes and also bigger than surface micromachined parts. The process steps for LIGA are shown in following figure.
A Figure Showing Process Steps for LIGA
Fig. 6: A Figure Showing Process Steps for LIGA
·         Fusion Bonding
To form complex & large structures, the process of fusion bonding (uses both bulk and surface micromachining) may be used. It entails building up a structure by atomically bonding various wafers. In this case, cavity is bulk etched in the bottom wafer. Then, second wafer is then bonded forming buried cavity. This is followed by patterning of DRIE masking material on the top wafer. Anisotropic etching is then carried out to release the microstructure, followed by removal of DRIE masking material to produce the final device.
A Figure Showing Process of Fusion Bonding used to form Larger, Complex Structures
Fig. 7: A Figure Showing Process of Fusion Bonding used to Form Larger, Complex Structures
 
MEMS-MATERIALS
The choice of a good material for MEMS application depends on its properties, but not so much on carrier mobility as in microelectronics. Actually, materials are selected more on mechanical aspect; small or controllable internal stress, low processing temperature, compatibility with other materials, possibility to obtain thick layer, patterning possibilities, etc. In addition, depending on the fieeld of application, the material often needs to have extra properties. RF MEMS requires material to have with small loss tangent, , optical MEMS may need a transparent substrate, BioMEMS will need bio-compatibility, sensing application may need materials to have piezoresistivity/piezoelectricity, etc. Commonly used Materials for MEMS are Si, SiO2, SiN, PolySi, Glass, Gold, Aluminium, etc.

MEMS Components

 

MEMS COMPONENTS - EXAMPLES

·         Micromachined High Q Inductors
Bulk micromachining has reduced the parasitics associated with conventional on-chip planar inductors, which used to lower their quality factor (Q).
Figure shows an example of Bulk-Micromachined Inductor Where the Substrate has been Eliminated from Spiral Trace
Fig. 8: Figure Shows an Example of Bulk-Micromachined Inductor where the Substrate has been Eliminated From Spiral Trace
Figure  shows an example of a bulk-micromachined inductor in which the substrate has been eliminated from underneath the spiral trace. Measured Qs range from 6 to 28 at frequencies from 6 to 18 GHz, with typical inductor A Figure showing an Example of use of Machine to create Solenoid - like Inductors
Fig. 9: A Figure Showing an Example of use of Machine to create Solenoid - Like Inductors
values around 1nH.
Similarly, machining has been exploited to create solenoid-like inductors above the substrate. Figure 5 shows an example of such an approach. A quality factor of 25.1 at 8.4 GHz and an inductance of 2.3 nH were obtained.
·         MEMS Varactors
A Figure Demonstrating Few Examples of Variants of MEMS based Varactors
Fig. 10: A Figure Demonstrating few Examples of Variants of MEMS Based Varactors
 
MEMS-based varactors  broadly are of two types — parallel plate and interdigitated capacitor. Some variants of the above types have also been demonstrated.  
In the parallel (two or three) plate approach, the top plate is suspended a certain distance from the bottom plate by suspension springs, and this distance is made to vary in response to the electrostatic force between the plates induced by an applied voltage
A Figure Illustrating the Interdigiated Approach of MEMS Varactor
Fig. 11: A Figure Illustrating the Interdigiated Approach of MEMS Varactor
 
In the interdigitated approach, the effective area of  the capacitor is varied by changing the degree of engagement of the fingers of comblike plates. Comb drive type actuators make use of this approach.
 ·         MEMS Switches
MEMS switches offer low insertion loss, high isolation and high linearity. Many switches, based on a number of actuation mechanisms and topologies, have been demonstrated. These include the Electrostatic, Piezoelectric, Thermal, Magnetic, Bi-Metallic (shape-memory alloy).
Structure RF MEMS (Shunt) Switches are shown in following figure.

A Figure Showing Structure of RF MEMS (Shunt) Switches

Fig. 12: A Figure Showing Structure of RF MEMS (Shunt) Switches

·         Cavity Resonators
The performance levels of macroscopic waveguide resonators may be approached using MEMS.  As an example, micro machined cavity resonator for X-band applications has been demonstrated to have unloaded Q of 506 for a cavity with dimensions 16× 32× 0.465 mm. This was just 3.8 percent lower than the unloaded Q obtained from a rectangular cavity of identical dimensions. A Figure Demonstrating an Example of Micro Machined Cavity Resonator for X-Band Applications'
Fig. 13: A Figure Demonstrating an Example of Micro Machined Cavity Resonator for X-Band Applications
 
·         Micromechanical Resonators
A Figure Illustrating Process Flow of Micromechanical Resonators

 

Fig. 14: A Figure Illustrating Process Flow of Micromechanical Resonators

Mechanical resonators are capable of exhibiting Q in the 10,000-to-25,000 range. Micromechanical resonators can also achieve this using vertical displacement resonator, in which a cantilever beam is set into a diving board-like vertical vibration in response to an electrostatic excitation, and the lateral displacement resonator, in which the motion is excited by exciting a comb-like structure. For higher frequencies, the Film Bulk Acoustic wave Resonator(FBAR) consisting of a layer of piezoelectric material disposed between can be used.
                                                                                   
·         Micromachined Gears
A Figure Showing Electrostatic Microengine Output Gear Coupled to a Double Level Gear Train
Fig. 15: A Figure Showing Electrostatic Microengine Output Gear Coupled to a Double Level Gear Train
 
Figure shows electrostatic microengine output gear coupled to a double level gear train that drives rack and pinion slider. This is fabricated using Sandia Ultra-planar Multi-level MEMS Technology(SUMMiT).
 
·         MEMS Rotary Motor
            A figure showing an electrostatic rotary motor fabricated using multi-user MEMS process
Fig. 16: A Figure Showing an Electrostatic Rotary Motor Fabricated using Multi-User MEMS Process
 
An electrostatic rotary motor has been fabricated using Multi-user MEMS process.
 

MEMS Packaging

MEMS PACKAGING
Packaging is very important for MEMS devices. Reliability, performance of a MEMS device largely depends on packaging and this is one of the factors which have affected the growth of MEMS technology. It should
• provide protection and be robust enough to withstand its operating environment.
• allow for environmental access and connections to physical domain.
• minimize EMI.
• dissipate generated heat.
• minimize stress from external loading.
• handle power from electrical connection leads without disruption.
 
Hermetic sealing is a universal requirement in packaging many MEMS devices, especially in micro fluidics and micro optical switches. Since MEMS device structures are typically very small, stresses and thermal effects, induced in the substrates during the packaging and interconnection steps, can adversely affect their mechanical performance. For devices like accelerometer which can be hermetically sealed, packaging is relatively much easier than the packaging for microfluidic sensors (e.g., pressure, flow, chemical, etc.). They need an interface for real world and hence, isolation of electrical interconnects from environment and protection of MEMS structure increases the complexity in these devices.
Types of MEMS packages used are
A diagram showing types of ceramic MEMS based packaging
Fig. 17: A Diagram Showing Types of Ceramic MEMS Based Packaging
o   Ceramic Packaging
·         Hermetic when sealed
·         High Young’s Modulus
·         Flip Chip or Wirebonding
A diagram showing plastic MEMS based packaging
Fig. 18: A Diagram Showing Plastic MEMS Based Packaging
o   Plastic Packaging
·         Not Hermetic
·         Postmolding
·         Premolding
A-diagram-showing-metal-MEMS-based-packaging
Fig. 19: A Diagram Showing Metal MEMS Based Packaging
o   Metal Packaging
·         Hermetic when sealed
·         Easy to assemble
·         Low Pin Count
 
Since the substrate on many ICs requires an electrical connection to bias it, sensor dies are usually mounted to a die attach pad in the package using a conductive bond. The die attach pad is typically joined to a metal lead frame with wire bonds providing the electrical connections to the lead frame fingers. Various bonding media include AuSi eutectic bonding, epoxy bonding (conductive or insulating depending on filler material) and glass usually loaded with silver. The package is subsequently formed by plastic moulding (as in the case of moulded plastic packages), sealed ceramic or metal caps (ceramic packages), or with a brazed metal cap to the base of a metal package.
 
Wire bonding (Ultrasonic and thermosonic) is the most common technique for electrically connecting the die. In flip-chip (FC) technology the chips are bonded face down to a substrate via bumps; materials include solder, gold, copper and nickel. On heating, the bump material melts and simultaneously forms all the electrical and mechanical connections between the chip and the substrate.
 
In order to protect MEMS devices, thin-film coatings of usually silicon dioxide or silicon nitride are deposited on the components using plasma enhanced chemical vapour deposition (PECVD); the process called passivation. It increases wear resistance and electrical insulation. Common encapsulants such as epoxies, silicones and polyurethanes are used to protect the sensor die against adverse influences from the environment (contaminants, mechanical vibration and shock).

MEMS Applications

 

COMMERCIAL PRODUCTS & APPLICATIONS
Following figure depicts the emergence of commercial MEMS products since first commercial product was launched in 1973 and this list is growing further.
Year
Product
Company
Category
1973
Piezoresistive Pressure Sensor
NS, Honeywell, Kistler
Mechanical Sensor
1984
Thermal Inkjet Printer
HP, Canon
Microfluidics
1987
Accelerometer(for Airbags)
Sensonor
Mechanical Sensor
1996
Video projector
TI
Optical MEMS
1998
Gyroscope
Micro-spectrometer
Bosch
Microparts
Mechanical Sensor
Optical MEMS
1999
Lab-on-a-chip
Optical Switch
Agilent/ Caliper
Sercalo
Microfluidics
Optical MEMS
2000
2D Optical Switch
OMM
Optical MEMS
2001
Microphone
Knowles
Mechanical Sensor
2002
Bolometer
3D Optical Switch
Ulis
Lucent
Optical MEMS Optical MEMS
2003
RF Switch
Teravicta
RF MEMS
2004
Inhaler nozzle
Microparts
Microfluidics
2005
Tunable Lens
Varioptic
Optical MEMS
2007
Silicon Oscillator
Discera
RF MEMS
2009
Scanner for projector
Microvision
Optical MEMS
And the list is growing on…
 
MEMS Accelerometer(used with airbags) which was initially developed to be crash sensor, has found its usage in lot of other products, viz., Camera, Binoculars, HDDs, Washing Machines, Helicopter toys, robots, etc.
 Some of the applications of MEMS products are:
1.      Automotive airbag accelerometer for automobiles & others
2.      Blood Pressure Sensors for medical applications
3.      Inkjet Printer Head for printers
4.      Digital Micromirror Device & Deformation mirror in optics
5.      Angular Rate Sensing Gyro &MEMS tuning fork gyro in avionics, defence, etc
6.      Micromachined piezoresisitive Manifold pressure sensor for automobile fuel injection systems
7.       Tire Sensors for automotive applications
8.      Grating Light Valve, a reflective display device.
9.      Low Insertion loss RF MEMS switches
10. Micromachined tunable fabry perot filters for infrared astronomyf
And many more.
 
Some of the emerging applications of MEMS include a BioMEMS device is the microtitreplate, classified as a ‘lab-on-a-chip’, product with dimensions of 20 mm x 37 mm x 3 mm enables automatic filling of 96 microwells by the use of capillary action. Implantable ‘pharmacy-on-a-chip’ devices to release drugs into the body from tiny chambers embedded in a MEMS device may be seeing the light of the day very soon. It will eliminate the need for needles or injections and will aid in delivery of insulin, hormones, chemotherapy drugs and painkillers. RF MEMS is growing very fast and is going to find its usage in mobile phones, steerable antennas, etc.

Comments

gud1......:)

NICE SUBJECTyes

Good article.

may I know the name of the sensor used? 

I am doing a project on MEMS based driving simulation, so could you please provide me the information for existing and proposed system of this project title. you can contact me on [email protected]