UNIT 5

Contents MEMS Computer Printer VCR Fax Machine NC machine

Introduction Micro Electro Mechanical

System(MEMS) is a device where microsensors and mechanical parts, along with signal processing circuits are integrated on a small piece of silicon.

Mechanical means they are basically the mechanical part i.e. actuation parts.

MICROSENSOR

MICROACTUATOR

MICROSTRUCTURE

MICROELECTRONICS

MEMS

•A typical MEMs system consists of a micro sensor which senses the environment and converts the environment variable into an electrical value. •The microelectronics processes the electrical signal and the micro actuator accordingly works to produce a change in the environment.

Distinctive features of the MEMS

Miniaturization

• Size is brought down for both sensors and actuators, and they are integrated together

Multiplicity

• Multiplicity is basically the multiple functions that are being made in a system

Microelectronics

• It integrates microelectronic control device with sensors and actuators.

Fabrication of MEMs device It involves the basic IC fabrication methods along with the

micromachining process involving the selective removal of silicon or addition of other structural layers.

Fabrication methods

Bulk Micromachining

Surface Micromachining

Molding( LIGA) Technique

Bulk Micromachining:

Introduction

• Bulk micromachining is a fabrication technique which builds mechanical elements by starting with a silicon wafer, and then etching away unwanted parts, and being left with useful mechanical devices. 

• Typically, the wafer is photo patterned, leaving a protective layer on the parts of the wafer that you want to keep. • The wafer is then submersed into a liquid etchant, like potassium hydroxide, which eats away any exposed silicon. 

Advantage:

• relatively simple and inexpensive fabrication technology• well suited for applications which do not require much complexity, and which are price sensitive.• cost less, • are highly reliable,• manufacturable, • there is very good repeatability between devices.

Application:

• Automotibiles• Medical Application

Surface Micromachining:Introduction

• Surface Micromachining builds devices up from the wafer layer-by-layer. While Bulk micromachining creates devices by etching into a wafer, • A typical Surface Micromachining process is a repetitive sequence of depositing thin films on a wafer, photo patterning the films, and then etching the patterns into the films.

Advantage:

• It is able to create much more complicated devices, capable of sophisticated functionality.• suitable for applications requiring more sophisticated mechanical elements

Disadvantage:

• Surface Micromachining requires more fabrication steps than Bulk Micromachining, and hence is more expensive. 

For the case of the structural level being silicon, and the sacrificial material being silicon dioxide, the final "release" process is performed by placing the wafer in Hydrofluoric Acid.

The Hydrofluoric Acid quickly etches away the silicon dioxide, while leaving the silicon undisturbed.

The wafers are typically then sawn into individual chips, and the chips packaged in an appropriate manner for the given application.

LIGA(Lithographie, Galvanoformung, Abformung): LIGA is a technology which creates small, but relatively

high aspect ratio devices using x-ray lithography. LIGA is a relatively inexpensive fabrication technology, and

suitable for applications requiring higher aspect ratio devices than what is achievable in Surface Micromachining.

The PMMA is then placed in a suitable etchant to remove the exposed areas, resulting in extremely precise, microscopic mechanical elements.

APPLICATION OF MEMS

Microsensors:

• (a) pressure sensors, (b) strain gauges, and (c) accerolometer  for the measuring of acceleration and (d) gyroscope for the measurement of rotation.

• car industry turbine of engine and power plant• airbag deployment sensor

Optical and micro-Mirrors:

• Involves the controlling and directing of the light band.• MEMS-based Micro-Mirror array is a likely candidate to replace LCD & LED as the dominant form of display technologies.  This

is due to the low-cost and high performance of the micro-mirrors. • Furthermore, due to the similar processes and facilities used in the fabrication of the MEMS micro-mirrors, it is relatively easy

to incorporate them with their controlling IC chip onto a single silicon substrate.• Digital Mirror Device (DMD) The DMD is a projection system based on a very large array of micromachined mirrors.  These

mirrors are integrated with on-chip CMOS microelectronics which control the position and operation of the mirrors.  A number of large screen projection systems currently on the market use this DMD chip as the heart of the system

Biomedical applications

• MEMS have the great potential in (a) the Biomedical Instruments and Analysis, and (b) Implants and Drug Delivery. • Miniaturization of surgical and diagnostic instruments are done for reasons like• (a)  cost reduction,• (b) less intrusive surgical procedures,• (c)  health concerns,• (d) reducing amount of test sample needed, e.g. blood,• (e)  speed of diagnosis,• (f)   patient recovery time and,• (g)  ease of usage

Micro and RF Switches

• MEMS-based devices can also be used to make high performance, high precision switches.  • These switches can be used for directing signals and to switch on or off micro devices.  • One of the commercialized switches can be found in the Optical industry and was developed

in 1999 by Marxer andSercalo for the directing of signals.  • One of the main advantages of the switch comes from its low rate of signal loss.

Current Challenges

Some of the obstacles facing organizations in the development of MEMS and Nanotechnology devices include:

Packaging• MEMS packaging is more challenging

than IC packaging due to :• the diversity of MEMS devices and

the requirement that many of these devices need to be simultaneously in contact with their environment as well as protected from the environment.

• Frequently, many MEMS and Nano device development efforts must develop a new and specialized package for the device to meet the application requirements. As a result, packaging can often be one of the single most expensive and time consuming tasks in an overall product development program.

Access to Fabrication• Most organizations who wish

to explore the potential of MEMS and Nanotechnology have little or no internal resources for designing, prototyping, or manufacturing devices, as well as little to no expertise among their staff in developing these technologies. Few organizations will build their own fabrication facilities or establish technical development teams because of the prohibitive cost.

Fabrication Knowledge Required• MEMS device developers must

have a high level of fabrication knowledge and practical experience coupled with a significant amount of innovative engineering skill in order to create and implement successful device designs. Often the development of even the most mundane MEMS device requires very specialized skills. Without this expertise and knowledge, at best device development projects can cost far more and take much longer. At worst, they can result in failure. 

NC MACHINE

In a Numerical Control machine, the program is fed to the machine through magnetic tapes or other such media. The original NC machines were essentially basic machine tools which were modified to have motors for movement along the axes.

In a Computer Numerical Controlled machine, the machines are interfaced with computers. This makes them more versatile in the sense that, suppose a change in dimension of a part is required. In a NC machine, you would have had to change the program in the tape and then feed it to the machine again. But in a CNC machine, you just change a variable in the computer and your modification is done.

in NC(numerical control) machine tools the part program is entered on the program tape in the form of punched holes. While in cnc machines the part program is entered into computer using input devices like keyboard, mouse, cd etc.

In nc machines the tape reader forms the machine control unit. While in cnc the computer and the controller forms the machine control unit

An NC machine is numerically controlled but has no memory storage and is run off of the "tape" each time the machine cycles. A CNC machine has memory storage and the program can be stored in its control. The NC machine was the fore-runner of the modern machine tool industry but is seldom used in todays manufacturing industry

Operating is easy in CNC machine because the input is given by keyboard and mouse whereas in NC machine input is given on tape and then the tape is inserted in the machine.

History

The first NC machines were built in the 1940s and 1950s, based on existing tools that were modified with motors that moved the controls to follow points fed into the system on punched tape.

These early servomechanisms were rapidly augmented with analog and digital computers, creating the modern CNC machine tools that have revolutionized the machining processes

Description Motion is controlled along multiple axes, normally at least two (X and

Y), and a tool spindle that moves in the Z (depth). The position of the tool is driven by direct-drive stepper motor or servo

motors in order to provide highly accurate movements, or in older designs, motors through a series of step down gears. 

Open-loop control works as long as the forces are kept small enough and speeds are not too great.

On commercial metal working machines, closed loop controls are standard and required in order to provide the accuracy, speed, and repeatability demanded.

As the controller hardware evolved, the mills themselves also evolved. One change has been to enclose the entire mechanism in a large box as a safety measure, often with additional safety interlocks to ensure the operator is far enough from the working piece for safe operation.

Most new CNC systems built today are 100% electronically controlled. CNC-like systems are now used for any process that can be described as

a series of movements and operations. These include laser cutting, welding, friction stir welding, ultrasonic welding, flame and plasma cutting, bending, spinning, hole-punching, pinning, gluing, fabric cutting, sewing, tape and fiber placement, routing, picking and placing, and sawing.

Examples of CNC machines

Mills Lathes Plasma cutters Electric discharge machining Wire EDM Water jet cutters