MEMS Sensors actuators

Topic: Sensors and actuators (MEMS), Conductivity of semiconductors, crystal planes and orientation. Presented by: Praveengouda Patil Univ Reg no: 09GAMB4009 Course: Advanced material Technology Semester: II

Contents

1) Introduction to Sensors and actuators

2) MEMS Technology Pressure Sensors

3) Applications of MEMS sensors/actuators in Auto industry

4) Actuators and types of actuators

5) Introduction to Semiconductors

6) Conductivity in Insulators, conductors and semiconductors

7) Change in conductivity of Si (semi conductor) atom due to addition of heat.

8) Crystals structure of semiconductors

9) Growth of semiconductor Crystals

10) Bibliography

SENSORS AND ACTUATORS

Introduction

An electronic control unit is in many ways similar to human brain. It received messages from various sources and after processing the information, it either instruct actuators to perform some physical action or it stores the data away in its memory for use at some time in the future.

Electronic sensors perform the information gathering role in this system. Each sensor feeds the electronic control module (ECU) with information that relates to some particular mechanical action or thermal effect. A sensor or transducer as it is called when it gives an output signal proportional to the physical quantity it is measuring converts physical actions it notices into either an analogue or digital electrical signal.

Sensors can be separated into two main classes, viz., active or self gathering and passive or modulating. The passive type requires an external energy source to drive it and the sensor acts only as an energy controller.

Block diagram of key pressure sensor components.

The pressure is measured by monitoring its effect on a specifically designed mechanical structure, referred to as the sensing element. The application of pressure to the sensing element causes a change in shape, and the resulting deflection (or strain) in the material can be used to determine the magnitude of the pressure.

The application of MEMS to the measurement of pressure is a mature application of micro machined silicon mechanical sensors, and devices have been around for more than 30 years. It is without doubt one of the most successful application areas, accounting for a large portion of the MEMS market. Pressure sensors have been developed that use a wide range of sensing techniques, from the most common piezoresistive type to high-performance resonant pressure sensors. The suitability of MEMS to mass-produced miniature high-performance sensors at low cost has opened up a wide range of applications. Examples include automotive manifold air and tire pressure, industrial process control, hydraulic systems, microphones, and intravenous blood pressure measurement. Normally the pressurized medium is a fluid, and pressure can also be used to indirectly determine a range of other measurands such as flow in a pipe, volume of liquid inside a tank, altitude, and air speed. Many of these applications will be highlighted in this chapter, demonstrating MEMS solutions to a diverse range of requirements.

MEMS Technology Pressure Sensors

1) Micromachined Silicon Diaphragms

MEMS pressure sensors typically employ a diaphragm as the sensor element. This is because of its compatibility with a range of bulk and surface silicon micromachining the most common fabrication method is anisotropic wet silicon etching, which allows good control over diaphragm dimensions and is a batch process capable of producing hundreds of devices simultaneously across a group of wafers When combined with a (100) wafer orientation, a wet potassium hydroxide (KOH) etch, for example, produces a rectangular diaphragm with sloping side walls that follow the (111) planes. A cross-section of a typical diaphragm is shown in Figure 6.14. Diaphragm thickness can be controlled by timing etch duration, or more precisely by using boron doping or electrochemical etch stops. Surface micromachining techniques are becoming increasing applied since they offer the opportunity for reduced device size and compatibility with integrated electronics.

Anisotropically etched silicon diaphragm

2) Piezoresistive Pressure Sensors The piezoresistive nature of silicon makes the use of diffused or implanted resistors an obvious and straightforward technique for measuring the strain in a micromachined silicon diaphragm. The piezoresistive effect of silicon was first exploited by bonding silicon strain gauges to metal diaphragms [7], but this is an unsatisfactory approach given the thermal mismatch between the metal, adhesive Layer and silicon. Diaphragms were first micromachined into the silicon itself by mechanical spark erosion and wet isotropic etching [8]. This was not a batch approach and therefore device costs were high. The use of anisotropic etching, anodic and fusion bonding, ion implanted strain gauges, and surface micromachining have since reduced the size and improved the accuracy of piezoresistive pressure sensors.

Cross-section view of a typical bulk micromachined piezoresistive pressure sensor.

3) Capacitive Pressure Sensors

Capacitive pressure sensors are typically based upon a parallel plate arrangement whereby one electrode is fixed and the other flexible. As the flexible electrode deflects under applied pressure, the gap between electrodes decreases and the capacitance increases. It consists of an anisotropically etched silicon diaphragm with the fixed electrode being provided by a metallized Pyrex 7740 glass die .The glass and silicon die were joined using anodic bonding at die level. This device demonstrated the main attractions of capacitive sensing, these being high sensitivity to pressure, low power consumption, and low temperature cross-sensitivity. The combination of materials and bonding mechanisms demonstrated remain a common choice for capacitive sensors All silicon devices fabricated by silicon fusion bonding and glass frit bonding have also been reported along with many surface-micromachined devices

4) Resonant Pressure Sensors Resonant pressure sensors typically use a resonating mechanical structure as a strain gauge to sense the deflection of the pressure-sensitive diaphragm. The resonant approach is more technically challenging for a number of reasons discussed below, but it does offer performance specifications beyond that achievable with piezoresistive and capacitive techniques

Cross-section through SOI capacitive pressure sensor

5) Microphones Microphones are a particular type of MEMS pressure sensor designed to transduce Acoustic signals into electrical output. MEMS technology is an attractive approach for mass-producing miniature devices in, for example, hearing aid applications.Microphone diaphragms, or membranes, should be highly sensitive,exhibit suitable dynamic behavior, and be packaged so as to remain insensitive to static pressures.

Use of Sensors (MEMS) in an Automobile includes

Crash Sensing for Airbag Control

Vehicle Dynamic Control

Rollover Detection

Antitheft Systems

Electronic Parking

Brake Systems

Vehicle Navigation

Systems

Exhaust gas sensor

Tire pressure sensor

Fuel sensor( level indicator and vapor pressure ) etc

ACTUATORS Actuator: Actuator is a device that produces motion (displacement). ACTUATOR TYPES: ROTARY ACTUATOR: The rotary actuator is a device use to alternate the rotated position of an object. Just like the human wrist the actuator enables the rotation of an object, except that rotary actuators are available in a wide variety of models with different - Sizes, Torques, Rotation angles. The energy for the rotation is delivered by pneumatic pressure. The rotary actuator converts the air pressure from a linear motion to a rotating motion.

LINEAR ACTUATOR: Electric actuators with an output rod that provides linear motion via a motor driven ball or ACME screw assembly. The actuator's load is attached to the end of screw, or rod, and is often unsupported. Electric actuators whose load is attached to a fully supported carriage. Rod less linear actuators provides linear motion via a motor driven ball screw, acme screw, or belt drive assembly. ELECTRICAL ACTUATOR: Electrical actuators use a motor to drive a combination of gears that generate the desired torque or thrust level.

Classification of Actuators Based on Operating Media Actuator Classified as PNEUMATIC ACTUATOR & HYDRAULIC ACTUATOR: Hydraulic actuators and pneumatic actuators use pressurized fluid such as air or hydraulic oil to produce Linear or Rotary motion in mechanical components. Though hydraulic and pneumatic power shares many characteristics in common, there are some key differences. For example, because hydraulic fluid is much less compressible than a gas, Hydraulic power is preferred over pneumatic when precise position control is required. On the other hand, pneumatic power has an edge in applications where the presence of hydraulic oil could cause problems (e.g. in food processing machines). Pneumatic systems are also typically less expensive to build than hydraulic.

Based on actuator action: SINGLE ACTING: cylinders would extend by pneumatic or hydraulic pressure on the ram, And retract by spring force or gravity.

DOUBLE ACTING: cylinders operate by fluid pressure in both directions: in other words, there is a pressure connection on both sides of the ram to force it one way or the other within the cylinder, depending on the valve setting. These actuators are therefore capable of a power stroke in both directions. Sometimes the piston is even fixed and the cylinder moves back and forth around it. ACTUATOR MOUNTING CONFIGURATION: CYLINDER TYPES ACCORDING TO MOUNTING STYLES

ACTUATOR INPUT VOLTAGE: The voltage supplied to the Actuator 115V, 230V ACTUATOR STROKE LENGTH: Maximum Travel length of Actuator in mm ACTUATOR INPUT PRESSURE: The pressure required to move the actuator from its upper Stop to rated travel in PSI.

Semiconductor A semiconductor is a solid that has electrical conductivity in between that of a conductor and that of an insulator, and can be controlled over a wide range, either permanently or dynamically. [1] Semiconductors are tremendously important in technology. Semiconductor devices, electronic components made of semiconductor materials, are essential in modern electrical devices. Examples range from computers to cellular phones to digital audio players. Silicon is used to create most semiconductors commercially, but dozens of other materials are used as well. Insulators Insulators have tightly bound electrons in their outer shell These electrons require a very large amount of energy to free them for conduction Let’s apply a potential difference across the insulator as shown below, The force on each electron is not enough to free it from its orbit and the insulator does not conduct Insulators are said to have a high resistivity / resistance.

Conductors Conductors have loosely bound electrons in their outer shell these electrons require a small amount of energy to free them for conduction Let’s apply a potential difference across the conductor as shown below, The force on each electron is enough to free it from its orbit and it can jump from atom to atom – the conductor conducts. Conductors are said to have a low resistivity / resistance.

Semiconductors Semiconductors have a resistivity/resistance between that of conductors and insulators Their electrons are not free to move but a little energy will free them for conduction The two most common semiconductors are silicon and germanium. The Silicon, Si, Atom Silicon has a valency of 4 i.e. 4 electrons in its outer shell each silicon atom Shares its 4 outer electrons with 4 neighboring atoms these shared electrons – Bonds – are shown as horizontal and vertical lines between the atoms.

If we extend this arrangement throughout a piece of silicon, we have the crystal Lattice of silicon this is how silicon looks when it is cold It has no free electrons – it cannot conduct electricity – therefore it behaves like an insulator

However, if we apply a little heat to the silicon an electron may gain enough energy To break free of its bond…It is then available for conduction and is free to travel Throughout the material Slide

Electron free from bond due to energy absorbed by heat

Let’s take a closer look at what the electron has left behind There is a gap in the Bond – what we call a hole this hole can also move…An electron – in a nearby bond – may jump into this hole…Effectively causing the hole to move…

Electron- hole pairs created due to more heat addition.

Take a piece of silicon… and apply a potential difference across it this set up an electric field throughout the silicon – seen here as dashed lines When heat is applied an electron is released and…The electron feels a force and moves in the electric field It is attracted to the positive electrode and re-emitted by the negative electrode.

Now, let’s apply some more heat…Another electron breaks free…And moves in the Electric field. We now have greater current than before…And the silicon has less resistance… If more heat is applies the process continues…More heat…More current… Less resistance…The silicon is acting as a thermistor. Its resistance decreases With temperature. This property of semiconductor is used in the device called Thermistor. The thermistor is a heat sensitive resistor when cold it behaves as an insulator i.e. it has a very high resistance when heated, electron hole pairs are released and are then available for conduction as has been described – thus its resistance is reduced

Thermistor and its Symbol

Thermistor are used to measure temperature, they are used to turn devices on, Or off, as temperature changes they are also used in fire-warning or frost-warning circuits. A semiconductor's intrinsic electrical properties are often permanently modified by introducing impurities by a process known as doping. Usually, it is sufficient to approximate that each impurity atom adds one electron or one "hole" that may flow freely. Upon the addition of a sufficiently large proportion of impurity dopants, semiconductors will conduct electricity nearly as well as metals. Depending on the kind of impurity, a doped region of semiconductor can have more electrons or holes, and is named N-type or P-type semiconductor material,

Respectively. Junctions between regions of N- and P-type semiconductors create electric fields, which cause electrons and holes to be available to move away from them, and this effect is critical to semiconductor device operation. Also, a density difference in the amount of impurities produces a small electric field in the region which is used to accelerate non-equilibrium electrons or holes. In addition to permanent modification through doping, the resistance of semiconductors is normally modified dynamically by applying electric fields. The ability to control resistance/conductivity in regions of semiconductor material dynamically through the application of electric fields is the feature that makes semiconductors useful. It has led to the development of a broad range of semiconductor devices, like transistors and diodes. Semiconductor devices that have dynamically controllable conductivity, such as transistors, are the building blocks of integrated circuits devices like the microprocessor. These "active" semiconductor devices (transistors) are combined with passive components implemented from semiconductor material such as capacitors and resistors, to produce complete electronic circuits. In most semiconductors, when electrons lose enough energy to fall from the conduction band to the valence band (the Energy levels above and below the band gap), they often emit light, a quantum of energy in the visible electromagnetic spectrum. This photoemission process underlies the light-emitting diode (LED) and the semiconductor laser, both of which are very important commercially. Conversely, semiconductor absorption of light in photo detectors excites electrons to move from the valence band to the higher energy conduction band, thus facilitating detection of light and vary with its intensity. This is useful for fiber optic communications, and providing the basis for energy from Solar cells. Semiconductors may be elemental materials such as silicon and germanium, or compound semiconductors such as Gallium arsenide and indium phosphide, or alloys such as silicon germanium or aluminium gallium arsenide. Band structure Band structure of a semiconductor showing a full valence band and an empty conduction band.

semiconductor crystal structures The most common crystal structure among frequently used semiconductors is the diamond lattice, shown in Figure 1 Each atom in the diamond lattice has a covalent bond with four adjacent atoms, which together form a tetrahedron. This lattice can also be formed from two face-centered-cubic lattices, which are displaced along the body diagonal of the larger cube in Figure 2by one quarter of that body diagonal. The diamond lattice therefore is a face-centered-cubic lattice with a basis containing two identical atoms.

Figure 1 : The diamond lattice of silicon and germanium

Compound semiconductors such as GaAs and InP have a crystal structure that is similar to that of diamond. However, the lattice contains two different types of atoms. Each atom still has four covalent bonds, but they are bonds with atoms of the other type. This structure is referred to as the zinc-blende lattice, named after zincblende (ZnS) as shown in Figure 2. Both the diamond lattice and the zinc-blende lattice are cubic lattices. A third common crystal structure is the hexagonal structure also referred to as the wurzite crystal structure, which is the hexagonal form of zinc sulfide (ZnS). Many semiconductor materials can have more than one crystal structure. A large number of compound semiconductors including GaAs, GaN and ZnS can be either cubic or hexagonal. SiC can be cubic or one of several different hexagonal crystal structures. The cubic crystals are characterized by a single parameter, the lattice constant a, while the hexagonal structures are characterized in the hexagonal plane by a lattice constant a and by the distance between the hexagonal planes, c.

Figure 2: The zinc-blende crystal structure of GaAs and InP

Growth of semiconductor crystals like all crystals, semiconductor crystals can be obtained

by cooling the molten semiconductor material. However, this procedure yields poly-

crystalline material since crystals start growing in different locations with a different

orientation. Instead when growing single-crystalline silicon one starts with a seed

crystal and dips one end into the melt. By controlling the temperature difference

between the seed crystal and the molten silicon, the seed crystal slowly grows. The

result is a large single-crystal silicon boule. Such boules have a cylindrical shape, in

part because the seed crystal is rotated during growth and in part because of the

cylindrical shape of the crucible containing the melt. The boule is then cut into wafers

with a diamond saw and further polished to yield the starting material for silicon device

fabrication.

Bibliography

1) Principles-of-Semiconductor-Devices by Zeghbroek.

2) MEMS mechanical sensors by Stephen Beeby and Graham Hensel.

3) MEMS Exchange http://www.mems-exchange.org/

4) MEMS Industry Group http://www.memsindustrygroup.org/

5) MEMS WWW Bookstore: http://mems.isi.edu/bookstore

6) An Introduction to MEMS Engineering; by Nadim Maluf